LibSyntax: Remove default switch case in `language_to_string()`

This way we can get a compiler error about unhandled new language cases,
instead of a possible crash when running a program.

Also added a missing case to make it build now.

Userland/Libraries/LibSyntax/Highlighter.cpp

@@ -155,13 +155,14 @@ StringView language_to_string(Language language)
         return "INI"sv;
     case Language::JavaScript:
         return "JavaScript"sv;
+    case Language::PlainText:
+        return "Plain Text"sv;
     case Language::Shell:
         return "Shell"sv;
     case Language::SQL:
         return "SQL"sv;
-    default:
-        VERIFY_NOT_REACHED();
     }
+    VERIFY_NOT_REACHED();
 }
 
 }