Fix CSS escapes in v2 ID and class selectors

[jdalton]

Summary

• Decode CSS escapes in ID and class selector tokens before building their matchers.
• Escape the decoded values for JavaScript regular expressions so punctuation stays literal.
• Add integration coverage for hex escapes, replacement characters, supplementary code points, and escaped punctuation.

Verification

• Pinned WPT dom/nodes/ParentNode-querySelector-escapes.html: 68/68 passing with the checkout engine (previously 27/68).
• npm test: 38/38 passing.
• git diff --check: clean.

npm run lint could not run because this checkout has no eslint executable installed.