LibJS: Let parser keep track of errors

Rather than printing them to stderr directly the parser now keeps a
Vector<Error>, which allows the "owner" of the parser to consume them
individually after parsing.

The Error struct has a message, line number, column number and a
to_string() helper function to format this information into a meaningful
error message.

The Function() constructor will now include an error message when
throwing a SyntaxError.

Libraries/LibWeb/DOM/Document.cpp

@@ -378,6 +378,7 @@ JS::Value Document::run_javascript(const StringView& source)
     auto parser = JS::Parser(JS::Lexer(source));
     auto program = parser.parse_program();
     if (parser.has_errors()) {
+        parser.print_errors();
         return JS::js_undefined();
     }
     dbg() << "Document::run_javascript('" << source << "') will run:";