feat: Adds a transient button component

[mister-ben]

Work in progress...

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... Aim is to be a generic button type to be extended.
Optionally takes focus when shown

Specific Changes proposed

Adds TransientButton class and styles.
Examples of use at https://deploy-preview-8629--videojs-preview.netlify.app/sandbox/transient-button.html

Open questions

• Should a button take focus when it is first shown? Accessibility / convenience
• Need to consider tab order as the buttons (as used in the demo) are after the control bar in the DOM but above it visually.
• Prevent buttons showing before playback?

[codecov]

Codecov Report

Attention: Patch coverage is 7.14286% with 26 lines in your changes are missing coverage. Please review.

Project coverage is 82.43%. Comparing base (b59a89b) to head (798b130).

Files	Patch %	Lines
src/js/transient-button.js	7.14%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8629      +/-   ##
==========================================
- Coverage   82.71%   82.43%   -0.28%     
==========================================
  Files         113      114       +1     
  Lines        7636     7664      +28     
  Branches     1835     1836       +1     
==========================================
+ Hits         6316     6318       +2     
- Misses       1320     1346      +26     

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