Add definition for composed live range

[dizhang168]

As discussed at TPAC 2024, the specification for the new getComposedRanges() API need the introduction of the new definition "Composed live range" [1], which should react to mutations. This PR attempts to define that new concept. Once landed, we will add the spec language that uses composed live range in the Selection API. An overview of the spec changes necessary can be found at [2].

[1] w3c/selection-api#2 (comment)
[2] https://github.com/dizhang168/shadow-dom-selection/blob/main/selection-api-spec-changes.md

• At least two implementers are interested (and none opposed):
  • Chromium: Positive
  • Webkit: Positive (Selection API: getComposedRanges mozilla/standards-positions#1055)
  • Gecko: Positive (Selection API: getComposedRanges WebKit/standards-positions#376)
• Tests are written and can be reviewed and commented upon at:
  • This PR introduces an internal concept as a Composed Live Range is not exposed directly to the user
  • getComposedRanges() function tests can be found at: https://wpt.fyi/results/selection/shadow-dom/tentative?label=experimental&label=master&aligned
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/40286116
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1430308
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=216325
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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