Add experimental "unified extraction" for parsing scripts and styles.

- Consolidate parsing scripts and styles from two methods to one.
- Unify the parse result object for scripts and styles.
- Parse and install scripts and styles from all fragments.
- For now, guard behind a `experimental-extract-unified` config.
- Improve parsing logic, handling HTML4/HTML5 formats and irregularities.
- Add tests.

Progress on #24.