Labels can have underscore.

Author: cristianoc

analysis/tests/src/Parser.res

@@ -128,3 +128,5 @@ let _ = x =>
   }
 
 let _ = 3 == 3 || 3 === 3
+
+let _ = (~_type_ as _) => ()

analysis/tests/src/expected/Parser.res.txt

@@ -1,5 +1,5 @@
 Parse tests/src/Parser.res
-structure items:36 diagnostics:0 
+structure items:37 diagnostics:0 
 Lident: M (0,7) Namespace
 Lident: C (1,9) Namespace
 Lident: Component (1,13) Namespace

grammars/rescript.tmLanguage.json

@@ -238,7 +238,7 @@
     "defaultIdIsVariable": {
       "patterns": [
         {
-          "match": "[A-Za-z][A-Za-z0-9]*",
+          "match": "[A-Za-z_][A-Za-z0-9_]*",
           "name": "variable"
         }
       ]