Support RegExp extended mode and comments

Summary

Implement the Stage 1 TC39 proposal: https://github.com/tc39/proposal-regexp-x-mode

Implement the current proposal's extended-mode grammar, comments, scoped changes, and character-class/escape boundaries.

Why

This proposal fits GocciaScript's modern ECMAScript surface and the 0.13.0 proposal roadmap. It should extend the engine's existing architecture rather than arrive as a userland shim.

Current behavior

Standard flags and scoped modifier groups exist, but extended whitespace/comment mode does not.

Expected behavior

Implement the current proposal's extended-mode grammar, comments, scoped changes, and character-class/escape boundaries.

Scope notes

Integrate in the RegExp compiler lexical layer so source offsets, diagnostics, Unicode sets, captures, and backreferences remain correct. Follow upstream spelling exactly; add literal/constructor tests and performance checks. Additive RegExp syntax: no engine flag.

Related bytecode roadmap: #871 and #875. Update the relevant language/built-in documentation and Goccia proposal inventory. Use the proposal repository and its current specification at implementation time; add focused project tests and matching pinned Test262/proposal coverage without regressions.

Acceptance criteria

• Parser/API surface, descriptors, evaluation order, errors, and observable protocol behavior match the current proposal.
• Implementation uses the identified shared engine seam without a duplicate compatibility shim.
• Focused positive, negative, abrupt-completion, and edge-case tests cover the proposal.
• Relevant project suites, pinned Test262 cases, formatting, and documentation checks pass.

Start with the TC39 RegExp extended-mode proposal and the engine's RegExp compiler lexical layer mentioned in the issue. Identify the shared engine seam, then review existing literal, constructor, project-suite, and pinned Test262/proposal tests. Done means proposal behavior, diagnostics, offsets, edge cases, performance checks, and relevant documentation all pass without regressions.