docs: Add instructions for local development
docs: Add instructions for local development

This commit adds instructions to DEVELOPER.md to explain

1. How to bring up the developer instance of VSCode
2. Build and install local `@angular/language-service`

angular/vscode-ng-language-service,angular/vscode-ng-language-service,angular/vscode-ng-language-service,angular/vscode-ng-language-service