Detect shebang for Groovy files

* Should match "#!groovy", "#!/usr/bin/env /path/to/groovy" & "#!/usr/bin/groovy" types
* Very useful for people editing Jenkinsfile configuration (which has no file extension)

extensions/groovy/package.json

@@ -8,6 +8,7 @@
 			"id": "groovy",
 			"aliases": ["Groovy", "groovy"],
 			"extensions": [".groovy", ".gvy", ".gradle"],
+			"firstLine": "^#!.*\\bgroovy\\b",
 			"configuration": "./language-configuration.json"
 		}],
 		"grammars": [{
@@ -20,4 +21,4 @@
 			"path": "./snippets/groovy.json"
 		}]
 	}
-}
+}