[MED] Make petal_components easier to open PRs for

[mplatts]

Problem:

It's hard for other devs to make PRs for petal_components. This because you have to run a seperate Phoenix project and reference petal_components either via a file path or in an umbrella project. Ideally it comes with it's own server.

Some other projects have one-file phoenix projects.

• https://github.com/tfwright/live_admin/blob/main/dev.exs
• https://gist.github.com/Gazler/b4e92e9ab7527c7e326f19856f8a974a