[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