fix: use LSP-compliant dynamic file watcher registration

[Swahjak]

Summary

• Moves file watching from client-side (VSCode-specific) to server-side using the LSP standard client/registerCapability request
• Removes client-side FileSystemWatcher from extension.ts
• Adds dynamic registration for didChangeWatchedFiles in onInitialized
• Handles file changes via connection.onDidChangeWatchedFiles
• Adds remove() and refresh() methods to DocumentCache

Benefits

• LSP-compliant: Follows LSP 3.17 specification
• Editor-agnostic: Works with any LSP-compliant editor (Neovim, Sublime, Emacs, etc.)
• Flexible patterns: Server controls which files to watch

Test plan

• Verify file watching works in VSCode after modifications
• Verify file watching works when files are created/deleted externally
• Test with another LSP-compliant editor if possible

Fixes #54