Parse mut-restricted field syntax

[dtolnay]

• Part of Tracking Issue for Restrictions rust-lang/rust#105077

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

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