Add Swift tree-sitter language support

[possibilities]

Summary

• Add tree-sitter queries and language mapping for Swift (.swift)
• Covers classes, structs, enums, actors, extensions, protocols, typealiases, functions, and initializers
• Swift reuses class_declaration for class/struct/enum/extension, differentiated by keyword child; extension uses user_type for name

Testing

pytest tests/test_swift_symbols.py tests/test_tree_sitter_languages.py tests/test_symbol_extraction_multilang.py -v