fix(progress control): Fix the video continuing to play when the user scrubs outside of seekbar (#4918)

Scrubbing inside the seekbar paused the player properly but scrubbing inside the progress control outside the seekbar, the player never paused. This meant that when you scrubbed, if you kept the mouse down but lingered for a moment, the player would continue playing until the mouse moved again.

This fixes it so that the seekbar mousedown and mouseup handlers are called when the progress control mousedown and mouseup handlers are triggered.

Middleware factories currently get run each time a source is set. Because middleware are assocated with a player, the factories should only run once per player.

This PR fixes it so that we associate a middleware instance with a middleware factory per player.
Each time a player is disposed, we will clear the cache of the middleware instances for that player.

Fixes #4677.

These changes address an issue where screen readers may repeatedly and redundantly read TextTrackMenuItem's control text on every texttrackchange event in some browsers.

The source of the problem is in the handleTracksChange() method of TextTrackMenuItem and its subclass OffTextTrackMenuItem, in which this.selected(true/false) gets called even if the selected state has not changed since its previous invocation.

Use ResizeObserver when available for better and more performant resizing information, otherwise, fall back to a throttled resize event on an iframe that's the size of the player.
Allows a video.js user to disable this by setting resizeManager: false as an option since the component will not be initialized.

Add a debounce util.

This reverts #4800 (e0ed0b5c) because we end up getting two playerresize events with the dimension methods now.

This will allow middleware to interact with calls to play() from the tech. This will require a method of indicating to middleware previously run that a middleware down the chain has terminated or stopped execution.

* Adds middleware mediator method that runs middleware from the player to the tech and a second time back up to the player. This category was created because play is both a setter(changes the playback state) and a getter(gets a native play promise if available). This also has the ability to tell whether a middleware has terminated before reaching the tech.
* Adds a middleware.TERMINATOR sentinel value that is available on the videojs object
* Adds play to the allowedMediators
* Adds paused to the allowedGetters
* Adds a sandbox example of a play mediator middleware

Add label meanings for collaborators to know when to put labels and to let contributors know what labels mean when they are applied.

Patching canPlayType on Android shouldn't be necessary on Android Chrome since it reports its ability to player HLS correctly.

* adding guide content

* fix link

* CR comments

* fixing a typo

* fixing example

Closes #4875

Fix #4853.

Fixes #2964.

fix: wrap audio change handler rather than bind so a player dispose doesn't affect other players (#4847)

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