Also adds the `github:` respecConfig key and an issues-summary appendix.

Also adds the `github:` respecConfig key and an issues-summary appendix.

Specifies expansion of MSE API to usage from DedicatedWorker contexts.

This is an MSEv2 feature tracked by issue w3c#175.

See also earlier WICG explainer [1].

This is a squashed commit of 23 original commits, notably:
* Adds `closing` issue and link to w3c#276.
* Adds the `github:` respecConfig key and an issues-summary appendix.
* Removes redundant 'Repository' otherLinks config (addition of 'github'
  to respecConfig has make the manual enumeration in the 'Repository'
  otherLinks portion of respecConfig redundant.)
* Adds subsections for each of the extended HTMLMediaElement's .seekable
  and .buffered attributes.
* Adds more normative detail to the .buffered extension.
* Specifies what .seekable and .buffered do when worker has terminated,
  and references w3c#277 for further discussion.
* Describes cross-context track object aliasing. AudioTrack, VideoTrack,
  TextTrack extensions' sourceBuffer attribute must be null in the
  Window context's alias of the track if the track is an alias to a
  worker-created track from MSE-in-Worker. This is to prevent any
  assumption that cross-context object references are somehow now
  allowed in MSE or HTMLMediaElement+MSE.
* Describes a MessageChannel-based cross-context communication mechanism
  in a new section, including how MessagePorts on that channel are given
  to each side (media element, MediaSource) for communicating state
  needed by algorithms. Updates multiple algorithms and methods to use
  this mechanism where necessary. Resolves w3c#279.
* Specifies how to detect this feature using new
  `canConstructInDedicatedWorker` attribute without requiring the
  detection happening in a DedicatedWorker context.
* Modernizes to use respec's new variable syntax (`|...|` with types
  inlined)) for related lines. Wider modernization is an editorial item
  that is out of scope of this feature.
* Modernizes to use internal slots for the new cross-context
  communication mechanism's channel and ports. Wider modernization is an
  editorial item out of scope of this feature.o

Note, multiple simultaneous changes to track's `selected`, `enabled` or
`hidden`/`showing` states could be in-flight from window to worker, and
from worker to window. A non-normative note might be good to add here to
warn API users of this possibility. See w3c#278.

Note, {Audio,Video,Track}{,List} IDL need to be exposed in
DedicatedWorker. This might need to be directly in the media element
spec. See w3c#280.

Note, a reference to an MSE-in-Worker example/demo would be good in
the merged PR. Such a demo exists already at [2].
[1] https://github.com/wicg/media-source/blob/mse-in-workers-using-handle/mse-in-workers-using-handle-explainer.md
[2] https://wolenetz.github.io/mse-in-workers-demo/mse-in-workers-demo.html

Specifies expansion of MSE API to usage from DedicatedWorker contexts.

This is an MSEv2 feature tracked by issue #175.

See also earlier WICG explainer [1].

This is a squashed commit of 23 original commits, notably:
* Adds `closing` issue and link to #276.
* Adds the `github:` respecConfig key and an issues-summary appendix.
* Removes redundant 'Repository' otherLinks config (addition of 'github'
  to respecConfig has make the manual enumeration in the 'Repository'
  otherLinks portion of respecConfig redundant.)
* Adds subsections for each of the extended HTMLMediaElement's .seekable
  and .buffered attributes.
* Adds more normative detail to the .buffered extension.
* Specifies what .seekable and .buffered do when worker has terminated,
  and references #277 for further discussion.
* Describes cross-context track object aliasing. AudioTrack, VideoTrack,
  TextTrack extensions' sourceBuffer attribute must be null in the
  Window context's alias of the track if the track is an alias to a
  worker-created track from MSE-in-Worker. This is to prevent any
  assumption that cross-context object references are somehow now
  allowed in MSE or HTMLMediaElement+MSE.
* Describes a MessageChannel-based cross-context communication mechanism
  in a new section, including how MessagePorts on that channel are given
  to each side (media element, MediaSource) for communicating state
  needed by algorithms. Updates multiple algorithms and methods to use
  this mechanism where necessary. Resolves #279.
* Specifies how to detect this feature using new
  `canConstructInDedicatedWorker` attribute without requiring the
  detection happening in a DedicatedWorker context.
* Modernizes to use respec's new variable syntax (`|...|` with types
  inlined)) for related lines. Wider modernization is an editorial item
  that is out of scope of this feature.
* Modernizes to use internal slots for the new cross-context
  communication mechanism's channel and ports. Wider modernization is an
  editorial item out of scope of this feature.o

Note, multiple simultaneous changes to track's `selected`, `enabled` or
`hidden`/`showing` states could be in-flight from window to worker, and
from worker to window. A non-normative note might be good to add here to
warn API users of this possibility. See #278.

Note, {Audio,Video,Track}{,List} IDL need to be exposed in
DedicatedWorker. This might need to be directly in the media element
spec. See #280.

There were some still unresolved cosmetic comments on this PR during
later portion of review that I'll follow-up on in a later PR.

Note, a reference to an MSE-in-Worker example/demo would be good in
the merged PR. Such a demo exists already at [2].
[1] https://github.com/wicg/media-source/blob/mse-in-workers-using-handle/mse-in-workers-using-handle-explainer.md
[2] https://wolenetz.github.io/mse-in-workers-demo/mse-in-workers-demo.html