LibSyntax: Add common_language_extension() function

The main motivation for this is to prefill an extension for user when
saving a new file.

Userland/Libraries/LibSyntax/Highlighter.h

@@ -33,6 +33,7 @@ struct TextStyle {
 };
 
 StringView language_to_string(Language);
+StringView common_language_extension(Language);
 
 class Highlighter {
     AK_MAKE_NONCOPYABLE(Highlighter);