feat: Add lossy conversion between QJS string and Rust string

[richarddd]

"🌍🌎🌏".slice(1) currently produces a broken surrogate pair. When converting to a rust string, we get UTF8 error.
QJS handles this by just printing the invalid utf16 surrogate pair as a replacement char.

This PR introduces a new method to_string_lossy that will render a utf8 replacement char for broken surrogate pairs.

[Sytten]

@richarddd I would argue that we should probably start using the new UTF-16 API instead quickjs-ng/quickjs#992 and do all the conversion on the rust side.

[richarddd]

Would that work for existing strings tho? It would be better if we could expose if the string is wide or not and use String::from_utf8_lossy or String::from_utf16_lossy

[Sytten]

We now have from and to utf16 from the engine, we should use that