Handle pub tuple fields in tuple structs

Handle pub tuple fields in tuple structs

The current implementation will throw a parser error for tuple structs
that contain a pub tuple field. For example,
```rust
struct Foo(pub (u32, u32));
```
is valid Rust, but rust-analyzer will throw a parser error.  This is
because the parens after `pub` is treated as a visibility context.
Allowing a tuple type to follow `pub` in the special case when we are
defining fields in a tuple struct can fix the issue.