On resolve error of `[rest..]`, suggest `[rest @ ..]`

When writing a pattern to collect multiple entries of a slice in a
single binding, it is easy to misremember or typo the appropriate syntax
to do so, instead writing the experimental `X..` pattern syntax. When we
encounter a resolve error because `X` isn't available, we suggest
`X @ ..` as an alternative.

```
error[E0425]: cannot find value `rest` in this scope
  --> $DIR/range-pattern-meant-to-be-slice-rest-pattern.rs:3:13
   |
LL |         [1, rest..] => println!("{rest:?}"),
   |             ^^^^ not found in this scope
   |
help: if you meant to collect the rest of the slice in `rest`, use the at operator
   |
LL |         [1, rest @ ..] => println!("{rest:?}"),
   |                  +
```

Fix #88404.

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