feat(tscn-to-bsn): switch to git dependency and add converter binary

Restructure the tool to use mingling from git with pathf feature,
add a dedicated tscn2bsn binary, and implement:

- Full TSCN syntax parser as a 1:1 replica of Godot's parsing logic
- AST representation with line numbers for error reporting
- Conversion command that reads TSCN, parses to AST, and serializes to
  RON
- Error handlers for IO, RON, and parse errors

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