LibJS: Make Parser::Error a standalone ParserError class

This allows us to forward declare it and reduce the number of things
that need to include Parser.h.

Userland/Libraries/LibJS/Forward.h

@@ -181,6 +181,8 @@ class Intrinsics;
 class Module;
 class NativeFunction;
 class ObjectEnvironment;
+class Parser;
+struct ParserError;
 class PrimitiveString;
 class PromiseCapability;
 class PromiseReaction;