Fix func keyword highlighting (#398)

syntaxes/GDScript.tmLanguage.json

@@ -89,6 +89,9 @@
         {
             "include": "#function-declaration"
         },
+        {
+            "include": "#function_keyword"
+        },
         {
             "include": "#any-method"
         },
@@ -506,6 +509,10 @@
                 }
             ]
         },
+        "function_keyword": {
+            "match": "func",
+            "name": "keyword.language.gdscript"
+        },
         "parameters": {
             "name": "meta.function.parameters.gdscript",
             "begin": "(\\()",