Add mix format into workflow

[SimonLab]

We can enhance the elixir files by using mix format.

For example we can run the command on a specific file with:

mix format lib/cs_guide_web/controllers/brand_controller.ex

What is the best way to add this formatting on new commits?

[nelsonic]

@SimonLab consider using https://github.com/dwyl/elixir-pre-commit 💡