Updates based on preload branch

[peaBerberian]

The last preload branch (#1407) is a good start for a preload API but is not yet finished and is already getting very big.

I noticed when working on it that several code updates made sense even without the Preload API.

So I decided to isolate those parts in a separate PR, where each commit details one type of update. The main theme is improving code readability:

• Rename segment_buffer file into the new segment_sink name

  We changed the name of classes from SegmentBuffer to SegmentSink but
  didn't actually update the file names. This is now done.

• Rely on compat version of TextTrack-related types in remove_cue.ts

• Provide more precize typing for the SegmentSinksStore's SegmentSinks

• ContentInit: Rename "settings" into "initSettings"

  To avoid the confusion between several concepts named "settings" in the
  MediaSourceContentInitializer, the settings provided to its
  constructor is now renamed to _initSettings (aligns with
  _initCanceller for its TaskCanceller initialized at instantiation).

• Rename and factorize some methods of the MediaSourceContentInitializer

  Some private methods of the MediaSourceContentInitializer were
  difficult to follow due to the heavy usage of inner functions with
  unclear names (such as recursivelyLoadOnMediaSource).

  I now tried to isolate some of those and rename the different methods
  called at content load so it makes more sense. Methods in order are:

  start > _setupInitialMediaSourceAndDecryption >
  _onInitialMediaSourceReady > _setupContentWithNewMediaSource >
  _startLoadingContentOnMediaSource
  

  Note: _setupContentWithNewMediaSource and
  _startLoadingContentOnMediaSource are still similarly named, though
  the former just has the task of setting up a reloading logic and then
  calling the latter.

  Note2: _startLoadingContentOnMediaSource is still a huge and complex
  method that we might improve on in the future.

• Isolate HTMLTrackElement clearing in its own private method

  This is a minor code update to make the NativeTextDisplayer's clean-up
  easier to follow by isolating the DOM-related (TextTrack,
  HTMLTrackElement, HTMLMediaElement) code inside its own private method.

• Make MainSourceBufferInterface code more readable isolating inner functions

  The callback called on the "updateend" and "error" events on a
  SourceBuffer were previously declared as arrow functions inside the
  constructor.

  This allowed to keep a clear JS context for the this keyword, but it
  made the MainSourceBufferInterface's constructor harder to read.

  I found it more readable to declare both callback as private methods
  instead, with the drawback of having to bind the this explictely.

• Fix SyncOrAsync util syncValue transfer

  Our SyncOrAsync util is used for cases where a task is >99% of the time
  synchronous, yet <1% of the time asynchronous, and where we don't want to
  incur any overhead/supplementary complexity of awaiting a Promise which
  inherently schedule a microtask when the value is most probably already
  there.

  It worked well for most usages, but it turns out that a task that starts
  as asynchronous would then always lead to the need to rely on Promises,
  even once the task is finished (basically, if it started as an "async
  value" it could never transform itself to a "sync value").

  This does not create any issue but we could gain some minuscule advantage
  (we could argue that SyncOrAsync's advantage is in itself
  minuscule) here by just relying on the value synchronously once the task is
  finished.