Support `textDocument/documentHighlight` message for usages of types

[gavinmorrow]

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

For example:

type Foobar {
//   ^^^^^^
  Foo
  Bar
}

type Qux(a) {
  // Cursor here (for example)
  //    ↓
  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
  }
}

Another example:

import foobar.{type Foobar, Foo, Bar}
//                  ^^^^^^

type Qux(a) {
  // Cursor here (for example)
  //    ↓
  Qux(Foobar, a)
  //  ^^^^^^
}

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

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

See #5427.