Add language service support for action.yml files (#275)

- Add validation, completion, hover, and document links for action.yml files
- Implement document type detection to route action.yml to action-specific handlers
- Add expression context for composite actions (inputs, steps, github, runner, etc.)
- Add schema validation for required fields, branding, and composite step requirements
- Support JavaScript (node20/node24), Docker, and composite action types
- Validate action references in composite action uses steps
- Add JSDoc comments to parser and template functions
- Refactor hover to use hoverToken consistently
- Fix lint errors and add return type annotations

.gitignore

@@ -4,6 +4,9 @@ lerna-debug.log
 node_modules
 .DS_Store
 
+# Nx cache (generated by Lerna/Nx)
+.nx/
+
 # Minified JSON (generated at build time)
 *.min.json