Parse final associated function syntax

- https://github.com/rust-lang/rust/issues/131179

Example: https://github.com/rust-lang/rust/blob/d39561c2d2b55985d3b6331cc52403e038e7fc8b/library/core/src/io/size_hint.rs#L43

```rust
pub trait SizeHint {
final fn size_hint(&self) -> (usize, Option) {
(self.lower_bound(), self.upper_bound())
}
}
```

Start with the Rust reference example in library/core/src/io/size_hint.rs and the linked Rust issue 131179, then trace how syn handles associated functions in traits. Done means syn accepts the shown final fn syntax and has coverage for it.

Written by the indexing model from the issue text.