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✨ Plugin API for to register backends and languages

### Added

- **External backend registration via `backend_module`** - External gems can now register
  their own pure Ruby backends using the same API as built-in backends. This enables gems
  like rbs-merge to integrate with `TreeHaver.parser_for` without modifying tree_haver:
  ```ruby
  TreeHaver.register_language(
    :rbs,
    backend_module: Rbs::Merge::Backends::RbsBackend,
    backend_type: :rbs,
    gem_name: "rbs"
  )
  # Now TreeHaver.parser_for(:rbs) works!
  ```
- **`Backends::PURE_RUBY_BACKENDS` constant** - Maps pure Ruby backend names to their
  language and module info. Used for auto-registration of built-in backends.
- **`TreeHaver.register_builtin_backends!`** - Registers built-in pure Ruby backends
  (Prism, Psych, Commonmarker, Markly) in the LanguageRegistry using the same API that
  external backends use. Called automatically by `parser_for` on first use.
- **`TreeHaver.ensure_builtin_backends_registered!`** - Idempotent helper that ensures
  built-in backends are registered exactly once.
- **`parser_for` now supports registered `backend_module` backends** - When a language
  has a registered `backend_module`, `parser_for` will use it. This enables external
  gems to provide language support without tree-sitter grammars:
  - Checks LanguageRegistry for registered `backend_module` entries
  - Creates parser from the backend module's `Parser` and `Language` classes
  - Falls back to tree-sitter and Citrus if no backend_module matches
- **RBS dependency tags in `DependencyTags`** - New RSpec tags for RBS parsing:
  - `:rbs_grammar` - tree-sitter-rbs grammar is available and parsing works
  - `:rbs_parsing` - at least one RBS parser (rbs gem OR tree-sitter-rbs) is available
  - `:rbs_gem` - the official rbs gem is available (MRI only)
  - Negated versions: `:not_rbs_grammar`, `:not_rbs_parsing`, `:not_rbs_gem`
  - New availability methods: `tree_sitter_rbs_available?`, `rbs_gem_av... (continued)

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