Explicitly match unit type inside generated setters

[aumetra]

This PR changes the usage of _ to () inside setters.

When implementing a setter, the code will expand to something like this:

let (field, _) = self.fields;

While this is fine, with Rust 1.73 Clippy will introduce a new pedantic lint ignored_unit_patterns.
This will result in warnings on each usage of the TypedBuilder macro.

With this one-line change, the expanded code will change to something like this:

let (field, ()) = self.fields;