add support for expert (#5036)

* add support for expert

* Refactor elixir expert linter from feedback

- Simplify configuration by removing executable lookup logic
- Update documentation to reflect current configuration options
- Rename test file and update assertions for the new configuration

* default to expert for the lsp

The install instructions included something I missed. It includes a
global install, where the default executable is just `expert`. I'll
default to that and review if I should should add a configuration for a
global config.

---------

Co-authored-by: Your Name <pmonson711@nfiindustries.com>

ale_linters/elixir/expert.vim

@@ -0,0 +1,12 @@
+" Author: Paul Monson <pmonson711@pm.me>
+" Description: Expert integration (https://github.com/elixir-lang/expert)
+
+call ale#Set('elixir_expert_executable', 'expert')
+
+call ale#linter#Define('elixir', {
+\   'name': 'expert',
+\   'lsp': 'stdio',
+\   'executable': {b -> ale#Var(b, 'elixir_expert_executable')},
+\   'command': '%e',
+\   'project_root': function('ale#handlers#elixir#FindMixUmbrellaRoot'),
+\})