feat(elixir): add Next LS

Next LS is a new language server for Elixir

Homepage: https://www.elixir-tools.dev/next-ls
GitHub: https://github.com/elixir-tools/next-ls

ale_linters/elixir/next_ls.vim

@@ -0,0 +1,17 @@
+" Author: Mitchell Hanberg <mitch@mitchellhanberg.com>
+" Description: Next LS  (https://github.com/elixir-tools/next-ls)
+
+call ale#Set('elixir_next_ls_executable', 'nextls')
+call ale#Set('elixir_next_ls_options', '--stdio')
+
+function! ale_linters#elixir#next_ls#GetCommand(buffer) abort
+    return '%e' . ale#Pad(ale#Var(a:buffer, 'elixir_next_ls_options'))
+endfunction
+
+call ale#linter#Define('elixir', {
+\   'name': 'next_ls',
+\   'lsp': 'stdio',
+\   'executable': {b -> ale#Var(b, 'elixir_next_ls_executable')},
+\   'command': function('ale_linters#elixir#next_ls#GetCommand'),
+\   'project_root': function('ale#handlers#elixir#FindMixUmbrellaRoot'),
+\})

test/linter/test_next_ls.vader

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+Before:
+  call ale#assert#SetUpLinterTest('elixir', 'next_ls')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(should set correct defaults):
+  AssertLinter 'nextls', '''nextls'' --stdio'
+
+Execute(should set next_ls options):
+  let b:ale_elixir_next_ls_executable = 'boo'
+  let b:ale_elixir_next_ls_options = '--spooky'
+
+  AssertLinter 'boo', '''boo'' --spooky'
+
+Execute(should set correct LSP values):
+  call ale#test#SetFilename('../test-files/elixir/umbrella_project/apps/app1/lib/app.ex')
+
+  AssertLSPLanguage 'elixir'
+  AssertLSPOptions {}
+  AssertLSPConfig {}
+  AssertLSPProject ale#path#Simplify(g:dir . '/../test-files/elixir/umbrella_project')