Support `textDocument/documentHighlight` message for definitions of variable

When triggering the message over the definition of a variable in a scope, highlight all instances of that variable.

For example:

```gleam
fn foo() {
  // Cursor here (for example)
  //  ↓
  let bar = 5
  //  ^^^
  let baz = bar * 2
  //        ^^^
  let qux = 3 + bar
  //            ^^^
  baz + bar + qux
  //    ^^^
}
```

See #5427.