Tracking issue for `bool::then_some`

Tracking issue for the `bool::then_some` method, which abstract the following pattern:

```rust
if some_condition {
    Some(t)
} else {
    None
}
```

`bool:then` has previously been stabilised as part of this feature gate: https://github.com/rust-lang/rust/pull/79299

RFC: https://github.com/rust-lang/rfcs/pull/2757
Implementation PR: https://github.com/rust-lang/rust/pull/64255