6733: Update attributes completion list r=jonas-schievink a=Veykril

Might be nice to have them grouped for readability/maintainability similar to how the [reference](https://doc.rust-lang.org/reference/attributes.html#built-in-attributes-index) does it but that would require the use of a `OnceCell` for sorting the entries back after construction.

Co-authored-by: Lukas Wirth <lukastw97@gmail.com>