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/ParserError.h

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2020, Stephan Unverwerth <s.unverwerth@serenityos.org>
+ * Copyright (c) 2021-2022, David Tuin <davidot@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <AK/String.h>
+#include <LibJS/SourceRange.h>
+
+namespace JS {
+
+struct ParserError {
+    String message;
+    Optional<Position> position;
+
+    String to_string() const;
+    String source_location_hint(StringView source, char const spacer = ' ', char const indicator = '^') const;
+};
+
+}