feat: add folding range provider

[NJichev]

Closes #56
Adds a folding range provider. For now it only works on do end blocks.
Things to consider:

• typespecs and docs and whether to fold them together with their respsective function - makes this PR a bit harder and maybe it's not necessary for starters?
• folding on fn -> end blocks - I think this should be added as well in the first version

[mhanberg]

The folds I added to nvim-treesitter currently consist of

[
  (anonymous_function)
  (stab_clause)
  (arguments)
  (block)
  (do_block)
  (list)
  (map)
  (tuple)
] @fold

So should at least do those.

Eventually it would be nice to fold consecutive alias Foo.Bar, as we have a lot of them in the Next LS code base, would be cool to fold them up.

[NJichev]

Thanks for the feedback, I'll go and implement them, will edit the original comment with a todo list for those.