feat: add range CSS parts

[luwes]

fix #921, fix #983

• Puts the background element in the track element for easier targeting the whole runnable track container.
• Adds CSS parts: thumb, track, background, highlight, pointer, progress.
• --media-range-track-color CSS var is removed from JS docs, this was left over from the old structure with CSS linear-gradient where it represented the color of the range section on the right of the thumb.

1. a smooth track height transition on hover looks like

  media-time-range::part(track) {
    transition: transform 0.2s;
    transform: scaleY(1);
  }

  media-time-range:hover::part(track) {
    transform: scaleY(1.5);
  }

2. adding border-radius to track highlight (buffer) and track progress elements

media-time-range::part(highlight),
media-time-range::part(progress) {
  border-radius: 9999px;
}

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.90%. Comparing base (3ea80df) to head (d6adb7c). Report is 112 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #990 +/- ## ========================================== - Coverage 78.55% 76.90% -1.66% ========================================== Files 59 50 -9 Lines 11080 12092 +1012 Branches 0 695 +695 ========================================== + Hits 8704 9299 +595 - Misses 2376 2768 +392 - Partials 0 25 +25 ```

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