Feat/environment variables

[nicolasguridi]

Context

Potassium-generated apps, like any other web app, rely on environment variables. Typically, these variables are accessed through class constants. However, this approach can lead to certain issues:

1. In some cases, an environment variable may be relevant to a group of classes rather than a single one. In such cases, it can be challenging to determine where to define the constant, resulting in it being placed arbitrarily in a class.
2. Since the constant's value is loaded when the class is defined, it can be difficult to mock it using RSpec.

What has been done

Summary:
A dedicated module for environment variables has been created. The benefits of this are:

1. Centralized location: all environment variables and their constants are now located in a single module, eliminating the need to arbitrarily choose a class to define them in.
2. Enhanced functionality: by using methods, it becomes possible to format or process environment variables as needed. This is a common requirement, given that environment variables are always strings.
3. Improved testability: by using methods, the environment variables are evaluated at runtime. This makes it easier to mock the constants containing them during testing.

Commits:

1. Added an environment variables module file to Potassium's assets
2. Created a recipe for this new module, including tests
3. Fixed some typos in the contributing guide
4. Updated changelog