Working directories are now set seperately from the commands so they
can later be swapped out when running linters over projects is
supported, and also better support filename mapping for running linters
on other machines in future.
Previously, elixir-ls would treat each sub-project within an umbrella as
standalone, which isn't desirable from a language server perspective.
Added ale#handlers#elixir#FindMixUmbrellaRoot, which locates the current
project's root and then continues searching upwards for a potential
umbrella project root. This literally looks just two levels up to keep
things simple while keeping in line with Elixir project conventions.
Use this new function to determine elixir-ls's LSP project root.
* FIX: use mix from the project root directory
* Move find root project function to autoloaded handlers
* add tests for #ale#handlers#elixr#FindMixProjectRoot
