Fix highlighting of `default` in textmate grammar. (#4038)

This is a modifier if it appears before an introducer and a control flow
keyword if it appears before `=>`. Use introducer highlighting if it's
neither.

utils/textmate/Syntaxes/carbon.tmLanguage.json

@@ -111,7 +111,11 @@
       "patterns": [
         {
           "name": "keyword.control.carbon",
-          "match": "\\b(break|case|continue|default|else|if|for|match|return|returned|then|while)\\b"
+          "match": "\\b(break|case|continue|else|if|for|match|return|returned|then|while)\\b"
+        },
+        {
+          "name": "keyword.control.carbon",
+          "match": "\\b(default)\\b(?=\\s*=>)"
         }
       ]
     },
@@ -143,12 +147,16 @@
       "patterns": [
         {
           "name": "storage.modifier.carbon",
-          "match": "\\b(abstract|default|extend|extern|final|private|protected|virtual)\\b"
+          "match": "\\b(abstract|extend|extern|final|private|protected|virtual)\\b"
         },
         {
           "name": "storage.modifier.carbon",
           "match": "\\b(base)\\b(?=\\s*class\\b)"
         },
+        {
+          "name": "storage.modifier.carbon",
+          "match": "\\b(default)\\b(?!\\s*=>)"
+        },
         {
           "name": "storage.modifier.carbon",
           "match": "\\b(export)\\b(?=\\s*import\\b)"