Fix String::operator+ memory leak

[sheepandshepherd]

Fixes godotengine/godot#32502.

new_string was adding a reference to this without removing it.

It looks like #307 also fixes this leak by cleaning up the way the underlying godot_string is accessed. Not sure if there's a reason it wasn't merged 2 months ago, but if the cleaner style is preferred, that one should be merged instead.

[2shady4u]

Looks good to me 👌

[akien-mga]

Thanks!