Accessibility audit/support

[benwiley4000]

We should try to make version 2 as accessible as possible. This means:

• Support for text tracks (may be somewhat involved to build an API around this)
• Support for screen readers... can a blind person read the UI easily? (only applicable to the default player implementation). Buttons should ideally have aria-label attributes explaining their function. Note that these should be swappable with other translations ideally.. (think about that for Right to left language support #247).
• Support for keyboard navigation for all functions - mostly need to make sure it's intuitive to adjust volume and progress via keyboard (only applicable to the default player implementation)

Resource: https://a11yproject.com/

[benwiley4000]

For text tracks:

Spun back into #108.

For screen readers:

Spun out as #399.

For keyboard navigation:

Again this will require some thought but might be more immediately feasible than text track support. Do we need to look at using a native element like range selector? Probably best to figure this out before we go into beta because it could affect the API for ProgressBar.

Spun out as #395.

[benwiley4000]

I forgot about #108. If we spin out text track support it can be tracked there.

[benwiley4000]

Keyboard navigation added in #396.

[benwiley4000]

Closing since all tasks are spun out into separate issues.