Folding Range

## Description

Port this feature from ElixirLS

https://github.com/elixir-lsp/elixir-ls/tree/master/apps/language_server/lib/language_server/providers/folding_range

Fold reasons:
- [ ] Indentation based
- [x] comment blocks (two or more consecutive lines)
- token pairs
- [x] do ... end
- [ ] with ... do
- [ ] for ... do
- [x] case ... do
- [x] fn ... end
- [ ] << ... >>
- [ ] { ... }
- [ ] [ ... ]
- [ ] ( ... )
- [x] heredocs (""" ... """)
- [x] multi-line strings
- [ ] charlist heredocs (do we need it?)
- [ ] multi-line charlist (do we need it?)
- [x] multi-line sigils
- [ ] HEEX sigils
- docs
- [x] moduledoc
- [x] typedoc
- [x] doc