Handle attribute string errors and escaped line continuations

[jdalton]

Tooling update

Rebased onto merged #212 as one commit. Sources and tests use .mts, Node tests use Vitest, and the published JavaScript paths stay unchanged. The original engine changes are preserved.

Local lint, formatting, Node and packed-package checks pass. All 41 WPT pages pass against both generated builds.

Change

Handle failed attribute matches through the normal syntax-error path and support escaped line continuations in quoted strings.

This follows the end-of-input discussion on #179. A backslash followed by LF, CR, CRLF or form feed continues a CSS string. A raw newline does not.

The parser consumes continuations before whitespace normalization, preserves hex-escape boundaries, and keeps valid end-of-input recovery. It rejects raw newlines, including form feed and a newline at the end of an unclosed string. Tag-prefixed match() calls no longer silently lose the invalid suffix.

The four former TODO cases are now ordinary passing tests. Validation:

• 163 attribute tests, four runtime tests and 11 adapter tests pass with jsdom 30.0.1. No TODOs or skips.
• 155 cases agree with Chromium 151.0.7922.34 for selection, first-match lookup and matching, on initial and cached calls.
• All 41 selected WPT pages pass with Add pinned WPT and isolated Node test tooling #167's baseline unchanged.

Run pnpm run test:attributes. For native-browser comparison, install Chromium and run pnpm run test:attributes:browser. See test/attribute-parse-error.md for details. One commit.