Fix rust generated code with struct of percent value

[ogoffart]

We wouldn't detect percent as a unit product in

slint/internal/compiler/generator/rust.rs

Lines 2418 to 2422 in 43c7f57

	if t.as_unit_product().is_some() {
	// number needs to be converted to the right things because intermediate
	// result might be f64 and that's usually not what the type of the tuple is in the end
	let t = rust_primitive_type(t).unwrap();
	quote!(as #t)

Percent is not strictly of percent unit because that unit doesn't combine with other unit (as per the comment in default_unit) But since it converts to number, it is then an unitless value. (scalar)

Fixes #5887