[WIP] goto def and find usage now backed by elixir-ls

[Trevoke]

With this, we can use:

• M-. to go to definition
• M-, to jump back

Things to test:

• [ ] Works with Elixir < 1.6
• [ ] Works with Elixir >= 1.6
• [ ] Works with Erlang 19
• [ ] Works with Erlang 20
• [ ] Works with Erlang 21

... And reasonable combinations of the above.

• [ ] behavior when trying to get to Erlang source code?
• [ ] behavior when trying to get to Elixir source code?
• [ ] Does the jump-back behavior work reasonably well?

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TODO:

• [ ] The tests need to pass