Replace if-let and while-let with `if let` and `while let`

[camelid]

I kind of like the look of if-let and while-let, but @varkor and I think that
it's probably clearer and more consistent with other error messages as `if let` and `while let`.

This will have to be changed in the compiler's error messages and the language
documentation (in src/doc/). Run rg -F -e '-let' compiler/ src/doc to find
out what to change.

Originally discussed in #79747 (comment).

[osa1]

@rustbot label: +E-easy

[TaKO8Ki]

I'm interested in working on this issue.
@rustbot claim