Add support for a number of new TLDs

[s3cur3]

This takes the changes I've been using in production on SleepEasy.

It adds support for the following new TLDs:

• .se
• .no
• .de
• .im
• .africa
• .io
• .com.au
• .com.br

It also adds a number of GitHub Actions checks:

• code quality (checking formatting, some basic Credo rules, and Dialyzer)
• compilation without warnings
• unit testing

It also bumps the minimum Elixir version to 1.12.