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.

Meta/Lagom/Fuzzers/FuzzJs.cpp

@@ -36,5 +36,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     auto lexer = JS::Lexer(js);
     auto parser = JS::Parser(lexer);
     parser.parse_program();
+    if (parser.has_errors())
+        parser.print_errors();
     return 0;
 }