LibRegex+LibJS: Change capture group names from a String to a FlyString

The parser now stores this as a FlyString everywhere, so consumers can
also use it as a FlyString.

Userland/Libraries/LibRegex/RegexMatch.h

@@ -487,7 +487,7 @@ public:
     }
 
     RegexStringView view { nullptr };
-    Optional<StringView> capture_group_name {};
+    Optional<FlyString> capture_group_name {};
     size_t line { 0 };
     size_t column { 0 };
     size_t global_offset { 0 };