Run `mix credo` on `GitHub Actions` for Code Consistency 😍

# Context

We have used 3^rd party code quality badges e.g. [`CodeClimate`](https://github.com/dwyl/repo-badges) in the past.
But haven't applied them to our `Elixir` projects ... until now! 🤷‍♂️

## Example

`while` reading: [https://github.com/cesium/safira](https://github.com/cesium/safira) discovered via @ruilopesm & @nelsonmestevao 🫶

we see they have the following badge:  prominently displayed:

The action is: [/cesium/safira/.github/workflows/style.yml](https://github.com/cesium/safira/blob/f4f415cf4f62ff6bed47947b021f91784d5bd724/.github/workflows/style.yml)

> **Note**: we are _using_ `credo` in several projects e.g: [`mvp`](https://github.com/dwyl/mvp/blob/d233698cd881305a42f08ae25686312f22d177e2/mix.exs#L105)
just not running the `mix` task on `GitHub Actions` or displaying a **badge** for "**Code Quality**".

# Todo ✅

+ [ ] Document the setup steps in a `code-style.md` file in this (`/learn-elixir`) repo. 📝
+ [ ] Create PR here in `/learn-elixir` 🆕
+ [ ] Add it to at least one repo, e.g: [`fields`](https://github.com/dwyl/fields) 🌻
+ [ ] Run the `mix credo --all --strict` on `localhost` to ensure it passes; 💻
+ [ ] Make any necessary changes. 🛠️
+ [ ] Add a `flat-square` badge to the repo. 🏷️
+ [ ] Create PR in the repo where the `style.yml` task is being added. :shipit:
+ [ ] Submit for review. 👀