feat(syntax): improve metadata header highlighting

[MayerTim]

Summary

Improves syntax highlighting for the generated metadata comment block in Watcom SQL files.

The metadata header now uses dedicated TextMate scopes for the metadata section markers, field names, separators, values, dates, authors, TODO placeholders, version values, and history entries. This makes the generated procedure metadata easier to read and visually separates it from regular comments.

Changes

• Added dedicated highlighting scopes for -- METADATA / -- METADATA END blocks
• Improved highlighting for metadata fields such as Description, Version, Author, Created, and Updated
• Added highlighting support for history entries
• Preserved compatibility with existing SQLovely-Metadata-Start / SQLovely-Metadata-Start marker behavior
• Added regression coverage for metadata scope matching

Testing

• Ran the extension test suite successfully
• Verified that the generated metadata block receives the expected syntax scopes