feat: Adds a transient button component

[mister-ben]

Description

Adds a TransientButton component for the types of button that are shown on top of the video briefly during playback and reappear when there is user activity. e.g. Unmute buttons, skip intro. It aims is to be a generic button type to be extended. Some basic styles are provided but kept light to not complicate customisation.
It's important to insert a transient button before the control bar for the tab order to make sense.

Optionally takes focus when shown.

Specific Changes proposed

Adds TransientButton component.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example: https://deploy-preview-8629--videojs-preview.netlify.app/sandbox/transient-button.html
  • Has no DOM changes which impact accessiblilty or trigger warnings (e.g. Chrome issues tab)
  • Has no changes to JSDoc which cause npm run docs:api to error
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.06%. Comparing base (6b468dd) to head (2c02312).

❗ Current head 2c02312 differs from pull request most recent head 7fc06a3

Please upload reports for the commit 7fc06a3 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8629      +/-   ##
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+ Coverage   83.01%   83.06%   +0.05%     
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  Files         119      120       +1     
  Lines        7993     8021      +28     
  Branches     1923     1924       +1     
==========================================
+ Hits         6635     6663      +28     
  Misses       1358     1358              

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