feat: migrate from Bun to npm for better compatibility

[timjonaswechler]

Summary

This PR migrates the project from Bun to npm for better compatibility and fixes all 4 open issues.

Migration Changes

• Replace Bun build system with standard TypeScript compilation
• Update package.json scripts to use npm and tsc instead of bun
• Add Jest configuration for testing with ESM support
• Update all ESM imports to use .js extensions for Node.js compatibility
• Remove Bun-specific dependencies and add npm equivalents
• Update README.md with npm-based installation and usage instructions
• Add TypeScript test configuration for Jest
• Maintain full functionality while using standard Node.js tooling

Issue Fixes

✅ Issue #1: Make version parameter explicitly required for search_symbols function

• Problem: Function marked version as optional but failed without it
• Fix: Made version parameter default "latest"
• Benefit: Eliminates confusing 404 errors

✅ Issue #2: get_source_code function consistently returns 404 errors

• Problem: Function used incorrect URL patterns for docs.rs source access
• Fix: Implemented multiple URL pattern attempts with fallback logic
• Benefit: Source code access now works with better error reporting

✅ Issue #3: get_crate_documentation returns "Documentation content not found"

• Problem: Used wrong URL path and CSS selectors for docs.rs documentation
• Fix: Corrected URL pattern and updated CSS selectors for docs.rs structure
• Benefit: Documentation retrieval now works correctly

✅ Issue #4: Improve error messages and API consistency

• Problem: Generic error messages made debugging difficult
• Fix: Added comprehensive parameter validation and specific error messages
• Benefit: Much better developer experience with clear, actionable error messages

Testing

• ✅ Build process works with npm run build
• ✅ Server starts correctly with npm run start
• ✅ Development mode works with npm run dev
• ✅ All 9 tests pass with npm test
• ✅ All MCP functions now work as expected

Benefits

• Better compatibility with standard Node.js tooling
• Easier for contributors who may not be familiar with Bun
• Uses widely adopted npm ecosystem
• Resolves all open functionality issues
• Maintains exact same functionality while fixing bugs