10699: internal: Make CompletionItem `label` and `lookup` fields `SmolStr`s r=Veykril a=Veykril
This replaces a bunch of String clones with SmolStr clones, though also makes a few parts a bit more expensive(mainly things involving `format!`ted strings as labels).
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>