[display] macro reification support

Several things I'd like to have to make the experience of using [expression reification](https://haxe.org/manual/macro-reification-expression.html) easier:

- [x] Auto-trigger completion on `$` when we're inside a macro (can be done with tokentree on the language server side)
- [ ] Show the full list with `$i{}` / `$v{}` etc in completion when inside `macro` (both after typing `$` or without it), including the docs from the manual page. Unclear if we need a new `CompletionItemKind` for this, perhaps `Keyword` is fine?
- [ ] Support signature help within the `{}` part (and we would auto-trigger signature help when selecting one of them from completion).
- [ ] Support hover on the "reification character" (e.g. `v` in `$v{}`) to show the associated docs.