Grammar: make async a keyword

Fixes #603

Author: cristianoc

Diff for: grammars/rescript.tmLanguage.json

@@ -5,7 +5,7 @@
   "repository": {
     "RE_KEYWORD_CONTROL": {
       "name": "keyword.control",
-      "match": "\\b(and|as|assert|await|constraint|downto|else|exception|external|for|if|in|lazy|mutable|rec|switch|to|try|when|while|with)\\b"
+      "match": "\\b(and|as|assert|async|await|constraint|downto|else|exception|external|for|if|in|lazy|mutable|rec|switch|to|try|when|while|with)\\b"
     },
     "RE_TO_DOWNTO_AS_LABELS": {
       "patterns": [