elixir-ls now recognizes umbrella projects

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.

ale_linters/elixir/elixir_ls.vim

@@ -16,6 +16,6 @@ call ale#linter#Define('elixir', {
 \   'lsp': 'stdio',
 \   'executable_callback': 'ale_linters#elixir#elixir_ls#GetExecutable',
 \   'command_callback': 'ale_linters#elixir#elixir_ls#GetExecutable',
-\   'project_root_callback': 'ale#handlers#elixir#FindMixProjectRoot',
+\   'project_root_callback': 'ale#handlers#elixir#FindMixUmbrellaRoot',
 \   'lsp_config_callback': ale#VarFunc('elixir_elixir_ls_config'),
 \})