Add `#[rustc_confusables]` attribute to allow targeted "no method" error suggestions on standard library types
After this PR, the standard library developer can annotate methods on e.g. `BTreeSet::push` with `#[rustc_confusables("insert")]`. When the user mistypes `btreeset.push()`, `BTreeSet::insert` will be suggested if there are no other candidates to suggest. This PR lays the foundations for contributors to add `rustc_confusables` annotations to standard library types for targeted suggestions, as specified in #59450, or to address cases such as #108437.
### Example
Assume `BTreeSet` is the standard library type:
```
// Standard library definition
#![feature(rustc_attrs)]
struct BTreeSet;
impl BTreeSet {
#[rustc_confusables("push")]
fn insert(&self) {}
}
// User code
fn main() {
let x = BTreeSet {};
x.push();
}
```
A new suggestion (which has lower precedence than suggestions for misspellings and only is shown when there are no misspellings suggestions) will be added to hint the user maybe they intended to write `x.insert()` instead:
```
error[E0599]: no method named `push` found for struct `BTreeSet` in the current scope
--> test.rs:12:7
|
3 | struct BTreeSet;
| --------------- method `push` not found for this struct
...
12 | x.push();
| ^^^^ method not found in `BTreeSet`
|
help: you might have meant to use `insert`
|
12 | x.insert();
| ~~~~~~
error: aborting due to previous error
```
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