feat: Make seek bar keyboard skip increment configurable

[mister-ben]

Description

Pressing the arrow and page keys seeks, currently by a set interval. This change makes this configurable.

Specific Changes proposed

Adds options that can be included in player options which set the increments to used.

{
  controlBar: {
    progressControl: {
      seekBar: {
        stepSeconds: 2,
        pageMultiplier: 4
      }
    }
  }
}

Defaults remain as 5 seconds for left/right and 12x multiplier for page up/down.

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 created (starter template on JSBin)
  • 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 83.83%. Comparing base (964c40e) to head (d115197).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8919      +/-   ##
==========================================
+ Coverage   83.42%   83.83%   +0.40%     
==========================================
  Files         120      120              
  Lines        8097     8096       -1     
  Branches     1944     1944              
==========================================
+ Hits         6755     6787      +32     
+ Misses       1342     1309      -33     

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[mister-ben]

Couple of points for feedback in particular:

• I'd considered making the increment for this and the seek buttons the same, but I'm not certain that would necessarily be what's wanted. Plus it would mean changing defaults in one the other.
• Making this a top level player option for convenience

[gkatsev]

I always forget that page up/down works.

[PlutaKatarzyna]

😄