Parse mut-restricted field syntax

- Part of https://github.com/rust-lang/rust/issues/105077

Example: https://github.com/rust-lang/rust/blob/d39561c2d2b55985d3b6331cc52403e038e7fc8b/tests/pretty/hir-mut-restriction.rs#L7

```rust
struct FooS {
pub(crate) mut(crate) x: i32,
mut(self) unsafe y: i32,
}
```

Start with the Rust issue linked in the body and the syntax example in tests/pretty/hir-mut-restriction.rs, especially the mut-restricted fields shown there. Trace how syn parses struct fields and add support for these forms; done means the example syntax parses successfully and existing parsing behavior remains intact.

Written by the indexing model from the issue text.