Modernize user_agent_parser

- remove P2 compatibility (e.g. str/bytes)
- remove never used `MatchSpans` methods (not sure what they were
  intended for)
- remove usage of `jsParseBits`, remove from caches and inner parsers,
  deprecate at the outer parser directly
- add a few types
- modernise code a bit (e.g. match group indexing, f-strings)

Author: masklinn