Move formatter from extension to language server

[wiyota]

Summary

• Move document formatting logic from extension client to language server
• Delete src/formatter.ts and create src/server/formatter.ts
• Add formatter configuration via initialization options
• Remove client-side formatting provider registration
• Update documentation to reflect formatter is now handled by language server
• Add comprehensive formatter tests with default prettier and disabled options
• Support workspace folder resolution and .prettierignore files in server
• Apply VS Code formatting options (tabSize, insertSpaces) to prettier

---

This change improves the architecture by centralizing formatting logic within the language server. As a result, formatting via the language server is now applied by default even in editors other than VS Code, and configuration (prettier or none) via initialization options is also possible.