LSP: Support finding references of modules

[gavinmorrow]

It would be nice to click on a module name and be able to find references of where it is used.

e.g. If in the following example, anytime the cursor was moved over mod and "go to reference" was activated, all the underlined areas would be found.

// -- mod.gleam
pub type Foo { Bar }

// -- app.gleam
import mod
//     ^^^
pub fn main() {
  let _: mod.Foo = mod.Bar
  //     ^^^       ^^^
}

// -- mod2.gleam
import mod
//     ^^^
pub fn fob() {
  mod.Bar
//^^^
}