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

[stephanieyzhang]

FormControlRange is a specialized, live AbstractRange subclass representing selection and caret positions within the value space of <input> and <textarea> elements. It enables range-based geometry operations for native form controls while preserving encapsulation by disallowing direct Range mutations.

This change adds getClientRects() and getBoundingClientRect() methods to the FormControlRange interface, aligning its behavior with Range for layout and rendering queries. It complements the DOM and HTML integrations of FormControlRange, 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 FormControlRange interface whatwg/dom#1404

• Corresponding HTML Spec PR: Integrate FormControlRange 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 FormControlRange 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