[css-highlight-api-1] Integrate OpaqueRange into Highlight and highlightsFromPoint

[stephanieyzhang]

Wires OpaqueRange into the Custom Highlight API so highlights can cover the value text of <input> and <textarea>.

• Highlight accepts OpaqueRange alongside Range and StaticRange.
• Defines when an OpaqueRange triggers repaints and when they're ignored.
• highlightsFromPoint() returns OpaqueRange hits inside <input> and <textarea> value text.
• Adds an example covering OpaqueRange in the main document and inside a shadow tree.

Chromium implementation:

• https://chromium-review.googlesource.com/c/chromium/src/+/7685965
• https://chromium-review.googlesource.com/c/chromium/src/+/7693796

Additional Links

• Explainer: https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/OpaqueRange/explainer.md
• DOM Spec PR: Add OpaqueRange interface whatwg/dom#1404
• HTML Spec PR: Integrate OpaqueRange hooks into text controls (input/textarea) whatwg/html#11741
• WHATWG Discussion Thread: OpaqueRange Interface whatwg/html#11478

[stephanieyzhang]

cc @ffiori and @dandclark for review -- couldn't add you directly as reviewers.