Improve parser robustness and highlighting performance

[romanbsd]

Summary

Adds regression tests and implements targeted robustness and performance improvements in the parser and highlighter.

Changes

• Fix repeated capture text being scoped at the wrong offset.
• Treat nullable pattern lists as empty lists instead of crashing.
• Add a deliberate UnsupportedError for unknown highlighter languages.
• Support combined theme fontStyle values such as bold italic underline.
• Cache theme scope fallback lookup.
• Avoid repeated string concatenation in bracket formatting.
• Share multiline scanner loops through common helpers.
• Replace an empty placeholder test file with focused regression coverage.

Validation

• flutter analyze
• flutter test