Add documentation about upgrading bitstyles versions; add versions showcase dev tool

[angelikatyborska]

This PR adds a document doc/bitstyles_version_compatibility.md, meant for devs developing bitstyles_phoenix. The goal is to make it easier for more people to help upgrade this project to work with newer bitstyles versions.

Additionally, this PR adds a script, scripts/generate_version_showcase.ex. The script generates static pages based on component stories. The static pages provide a preview for each component with each supported bitstyles version. The goal was to have a way to open the same component in different bitstyles versions side by side, so that we can compare the differences. This should make it easier to ensure components do not degrade in older bitstyles versions because of bugs introduced by default version upgrades.

This script is very experimental, the code quality is less than ideal, and it ignores the live/alpine3 components. Still, its output was very helpful for me to understand the bitstyles version changes, so I hope you'll accept it 😇

[angelikatyborska]

I extended the story macro to allow generating expected values per version:

  story(
    "An error tag",
    '''
        iex> assigns = %{}
        ...> render ~H"""
        ...> <.ui_error error={{"Foo error", []}} />
        ...> """
    ''',
    "4.3.0": '''
        """
        <span class="u-fg-warning">
          Foo error
        </span>
        """
    ''',
    "3.0.0": '''
        """
        <span class="u-fg--warning">
          Foo error
        </span>
        """
    '''
  )

This way, we can start accumulating per-version unit tests in the doctests that will give us more confidence when upgrading.

The downside is: the tests are now synchronous. The bitstyles version change happens by setting a global config option, so async tests would override one another.

[andreasknoepfle]

The downside is: the tests are now synchronous. The bitstyles version change happens by setting a global config option, so async tests would override one another.

We could put the override version into the process dict instead. Like this we could still have async test:
https://hexdocs.pm/elixir/1.12.3/Process.html#put/2

Love the syntax btw

[angelikatyborska]

The tests are back to asynchronous 😁 I keep forgetting about the process dictionary being available

[andreasknoepfle]

💚