Implement String.dedent

Summary

Implement the Stage 2 TC39 proposal: https://github.com/tc39/proposal-string-dedent

Implement current multiline indentation removal, substitutions, raw/cooked handling, line terminators, blank lines, and invalid escapes.

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

Template literals, tagged raw/cooked segments, and String.raw exist; String.dedent is absent.

Expected behavior

Implement current multiline indentation removal, substitutions, raw/cooked handling, line terminators, blank lines, and invalid escapes.

Scope notes

Build on GlobalString and template-object machinery, preserving template identity and canonical LF behavior across Windows/Unix. Add descriptors, cross-platform tests, docs, and inventory. Additive API: no 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 current TC39 String.dedent proposal and inspect the existing GlobalString and template-object machinery, including String.raw and tagged raw/cooked handling. Done means matching the proposal’s behavior and descriptors, with focused cross-platform tests, pinned Test262 or proposal coverage, documentation, inventory updates, and passing project checks.