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
