Support generic functions

[Brendonovich]

fn command(arg; tauri::AppHandle<impl tauri::Runtime>) no workey

[oscartbeaumont]

Right now Specta requires all generics for a function to be hardcoded to a specific type when exporting but it would maybe be nice if that wasn't a requirement and it could end up in TS as generics. That being said how do Rust generic bounds translate into Typescript bounds is a really big question here.

[oscartbeaumont]

We also don't support impl Runtime or the like as an argument because we don't ever know the concrete type of the function so it can't be exported.