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.

test/command_callback/test_elixir_ls_command_callbacks.vader

@@ -22,12 +22,12 @@ Execute(should configure elixir-ls release location):
   AssertLinter 'boo/language_server.sh', 'boo/language_server.sh'
 
 Execute(should set correct LSP values):
-  call ale#test#SetFilename('mix_paths/wrapped_project/lib/app.ex')
+  call ale#test#SetFilename('elixir_paths/umbrella_project/apps/app1/lib/app.ex')
 
   AssertLSPLanguage 'elixir'
   AssertLSPOptions {}
   AssertLSPConfig {}
-  AssertLSPProject ale#path#Simplify(g:dir . '/mix_paths/wrapped_project')
+  AssertLSPProject ale#path#Simplify(g:dir . '/elixir_paths/umbrella_project')
 
 Execute(should accept configuration settings):
   AssertLSPConfig {}

test/command_callback/test_elixir_mix_command_callbacks.vader

@@ -18,9 +18,16 @@ Execute(The default mix command should be correct):
   \ . g:env_prefix
   \ . 'mix compile %s'
 
-Execute(The FindMixProjectRoot should detect the project root directory via mix.exs):                                                  
-  silent execute 'file ' . fnameescape(g:dir . '/elixir_paths/mix_project/lib/app.ex')                                               
+Execute(FindMixProjectRoot should detect the project root directory via mix.exs):
+  silent execute 'file ' . fnameescape(g:dir . '/elixir_paths/mix_project/lib/app.ex')
 
-  AssertEqual                                                                  
-  \ ale#path#Simplify(g:dir . '/elixir_paths/mix_project'),     
+  AssertEqual
+  \ ale#path#Simplify(g:dir . '/elixir_paths/mix_project'),
   \ ale#handlers#elixir#FindMixProjectRoot(bufnr(''))
+
+Execute(FindMixUmbrellaRoot should detect the umbrella root directory via mix.exs):
+  silent execute 'file ' . fnameescape(g:dir . '/elixir_paths/umbrella_project/apps/app1/lib/app.ex')
+
+  AssertEqual
+  \ ale#path#Simplify(g:dir . '/elixir_paths/umbrella_project'),
+  \ ale#handlers#elixir#FindMixUmbrellaRoot(bufnr(''))