feat(lsp): Add document symbols provider

[mhiro2]

Summary

• Add lsp.symbols_document provider to display document symbols in the current buffer
• Support both DocumentSymbol (hierarchical) and SymbolInformation (flat) LSP response formats

Changes

• b68ad2a : feat(lsp): add document symbols provider
  • Add document_symbol_result_mapper to handle LSP textDocument/documentSymbol responses
  • Add append_document_symbol helper for recursive symbol tree flattening
  • Extend create_provider to accept custom result_mapper parameter
  • Support both DocumentSymbol (with range/selectionRange) and SymbolInformation (with location)
    formats
• f5028c8 : test(lsp): add symbols document coverage
  • Test hierarchical DocumentSymbol parsing with nested children
  • Test SymbolInformation format handling
  • Test warning when no LSP clients attached
  • Add same-location filter test for lsp.symbols_document
• 280013d : docs(lsp): document symbols provider usage
  • Add lsp.symbols_document to provider list in help documentation
  • Update README example and provider list