Add labels when showing a hovered function type

[giacomocavalieri]

This PR closes #3507 by adding labels to function types that are displayed when hovering on something. This works for constructors, imported functions, functions and function heads!

[lpil]

To me this introduces a new syntax which is a mix of the expression syntax and the type syntax, and it implies that labels are part of the type. Maybe it'd look more like the existing syntax if it included the function name?

Another thing is that for some expressions when you hover it shows the type at the top, but for these it does not. I'm not a big fan of this inconsistency. Is there a more definition-like format we could use for the others too?

[giacomocavalieri]

Maybe it'd look more like the existing syntax if it included the function name?

So what would it show in this example?

fn wibble(a: Int, label b: String) {}

pub fn main() {
  let var = wibble
//     ^ Cursor here:
//         var(Int, label: String)
}

Another thing is that for some expressions when you hover it shows the type at the top, but for these it does not.

Yeah I didn't think too much about it, not really sure. If the goal is to always keep a type you can copy-paste into Gleam code then I don't see a meaningful way to show labels for functions