Rollup merge of #123107 - avandesa:vec_pop_if, r=joboet

Implement `Vec::pop_if`

This PR adds `Vec::pop_if` to the public API, behind the `vec_pop_if` feature.

```rust
impl<T> Vec<T> {
    pub fn pop_if<F>(&mut self, f: F) -> Option<T>
        where F: FnOnce(&mut T) -> bool;
}
```

Tracking issue: #122741

## Open questions

- [ ] Should the first unit test be split up?
- [ ] I don't see any guidance on ordering of methods in impl blocks, should I move the method elsewhere?

library/alloc/src/lib.rs

@@ -170,6 +170,7 @@
 #![feature(unicode_internals)]
 #![feature(unsize)]
 #![feature(utf8_chunks)]
+#![feature(vec_pop_if)]
 // tidy-alphabetical-end
 //
 // Language features: