Add lexical for linting elixir projects (#4580)

ale_linters/elixir/lexical.vim

@@ -0,0 +1,19 @@
+" Author: Axel Clark <axelclark@pm.me>
+" Description: Lexical integration (https://github.com/lexical-lsp/lexical)
+
+call ale#Set('elixir_lexical_release', 'lexical')
+
+function! ale_linters#elixir#lexical#GetExecutable(buffer) abort
+    let l:dir = ale#path#Simplify(ale#Var(a:buffer, 'elixir_lexical_release'))
+    let l:cmd = has('win32') ? '\start_lexical.bat' : '/start_lexical.sh'
+
+    return l:dir . l:cmd
+endfunction
+
+call ale#linter#Define('elixir', {
+\   'name': 'lexical',
+\   'lsp': 'stdio',
+\   'executable': function('ale_linters#elixir#lexical#GetExecutable'),
+\   'command': function('ale_linters#elixir#lexical#GetExecutable'),
+\   'project_root': function('ale#handlers#elixir#FindMixUmbrellaRoot'),
+\})