(RDY) Parse correct locations extension expressions sugar

[IwanKaramazow]

Fixes #2143

[jordwalke]

You are awesome. Did you test via linking a local build of Reason?

[andreypopp]

I did test it on https://github.com/andreypopp/reason-ppx-let-merlin and seeing the same behaviour as previously. Did I missing something? refmt and ocamlmerlin-reason point to local builds.

[IwanKaramazow]

@jordwalke, using esy link to test this out locally
Still WIP, the locations under extension sugar need to be revised...

[IwanKaramazow]

@jordwalke this is ready.

Locations where missing in parts of the extension expression, resulting in Merlin have a very bad time.
Fixed all locations and streamlined the behaviour in terms of what the stock Ocaml parser does.

[andreypopp]

Just tested this and it works great! Autocomplete also started working for me (previously bindings introduced by let%bind weren't visible). Thank you @IwanKaramazow!

[jordwalke]

🎉
Thanks, this was the main blocker for converting more of the esy codebase (parts that use async operations)
Thank you, as always.

[zindel]

This works like a charm within the fastpack codebase! Thank you so much, @IwanKaramazow, this makes a huge difference!