change let coloring to keyword, not control

Author: zth

grammars/rescript.tmLanguage.json

@@ -15,7 +15,11 @@
     },
     "RE_KEYWORDS": {
       "name": "keyword.control",
-      "match": "\\b(and|as|assert|constraint|downto|else|exception|external|false|for|if|in|include|lazy|let|module|mutable|of|open|rec|switch|to|true|try|type|when|while|with)\\b"
+      "match": "\\b(and|as|assert|constraint|downto|else|exception|external|false|for|if|in|include|lazy|module|mutable|of|open|rec|switch|to|true|try|type|when|while|with)\\b"
+    },
+    "RE_LET": {
+      "name": "keyword",
+      "match": "\\b(let)\\b"
     },
     "RE_LITERAL": {
       "name": "constant.language",
@@ -91,6 +95,9 @@
       "patterns": [
         {
           "include": "#RE_KEYWORDS"
+        },
+        {
+          "include": "#RE_LET"
         }
       ]
     },