build: ignore ngPackagr temporary files

ngPackagr may leave some .ng_pkg_build files after an error
we may encounter this kind of errors when contributing to build_ng_packagr

Author: nlm-pro

.gitignore

@@ -16,6 +16,7 @@ node_modules/
 tmp/
 npm-debug.log*
 yarn-error.log*
+.ng_pkg_build/
 
 # Mac OSX Finder files.
 **/.DS_Store