Support `textDocument/documentHighlight` message for definitions of types

[gavinmorrow]

When triggering the message over the definition of a type in a file, highlight all instances of that type.

For example:

// Cursor here (for example)
//    ↓
type Foobar {
//   ^^^^^^
  Foo
  Bar
}

type Qux(a) {
  Qux(Foobar, a)
  //  ^^^^^^
}

const foo: Foobar = Foo
//         ^^^^^^

fn baz(foo: Foobar) -> Foobar {
//          ^^^^^^     ^^^^^^
  let quux: Foobar = foo
  //        ^^^^^^
  let qux: Qux(Foobar) = Qux(quux, foo)
  //           ^^^^^^
  case qux {
    Qux(Foo, Foo) -> Bar
    Qux(Bar, Bar) -> Foo
    _ → Foo
  }
}

See #5427.

[lpil]

Is this a duplicate of #5445?

[Wilbert-mad]

Basically, the same thing. Should close all but one and rename it simply "Support textDocument/documentHighlight"

[ankddev]

@Wilbert-mad Initially, @lpil adviced to close single big issue and open smaller ones for different features