[cssom-view-1] Add PlainTextRange geometry APIs (getClientRects/getBoundingClientRects)

[stephanieyzhang]

PlainTextRange is a specialized, live AbstractRange subclass representing selection and caret positions within the value space of <input>/<textarea> and other elements whose specifications define that they support plain text ranges. It enables range-based geometry operations for those elements while preserving encapsulation by disallowing direct Range mutations.

This change adds getClientRects() and getBoundingClientRect() methods to the PlainTextRange interface, aligning its behavior with Range for layout and rendering queries. It complements the DOM and HTML integrations of PlainTextRange, ensuring consistent geometry retrieval across editing and selection APIs.

• Explainer: https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/FormControlRange/explainer.md

• WhatWG discussion: FormControlRange Interface whatwg/html#11478

• Corresponding DOM Spec PR: Add PlainTextRange interface whatwg/dom#1404

• Corresponding HTML Spec PR: Integrate PlainTextRange hooks into text controls (input/textarea) whatwg/html#11741

• WPT tests are written and can be reviewed and commented upon at:

  • https://wpt.fyi/results/dom/ranges/tentative?label=master&label=experimental&aligned
  • PR to make tests non-tentative: Make PlainTextRanges tests non-tentative web-platform-tests/wpt#55014

• At least two implementers are interested (and none opposed):

  • Microsoft Edge
  • WebKit: FormControlRange - Live ranges for <input> and <textarea> WebKit/standards-positions#541
  • Mozilla: FormControlRange - Live ranges for <input> and <textarea> mozilla/standards-positions#1289

• Implementation bugs are filed:

  • Chromium: https://issues.chromium.org/issues/421421332
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1990256
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=299375

Current built changes: Overview.html