Fix constant syntax highlighting. (#222)

Author: floitsch

vscode/syntaxes/toit.tmLanguage.json

@@ -548,7 +548,7 @@
         },
         {
           "name": "constant.numeric.capitalized_user_constants.toit",
-          "match": "\\b[A-Z][A-Z0-9_]+\\b"
+          "match": "\\b_*[A-Z](-?[A-Z0-9_])+\\b"
         }
       ]
     },

vscode/syntaxes/toit.tmLanguage.yaml

@@ -335,7 +335,7 @@ repository:
     - name: constant.language.toit
       match: \b(null|true|false)\b
     - name: constant.numeric.capitalized_user_constants.toit
-      match: \b[A-Z][A-Z0-9_]+\b
+      match: \b_*[A-Z](-?[A-Z0-9_])+\b
 
   number:
     name: constant.numeric.toit