Update Mutant configuration

[jgarber623]

Description

I'll admit to being fairly new to mutant and mutation testing in general. I did notice, though, that the README and bin/run_mutant shell script didn't jive with recent(ish?) changes to that gem.

As-is, bundle install-ing this gem's dependencies and calling bin/run_mutant threw a number of errors. Seems there've been quite a few changes in mutant:

• run is the appropriate subcommand
• a usage flag is now required
• the --use flag is now --integration
• etc. etc. etc.

This PR refactors the existing configuration to align with that project's current version by adding a .mutant.yml file, removing this project's custom shell script, and updating the documentation.

Testing

1. git switch update-mutant-configuration
2. bundle install
3. bundle exec mutant run

I don't know enough to draw conclusions from the output of the test run, though. But, this change should be okay since mutation testing isn't run as part of ruby.yml GitHub Action. Maybe it should be? I dunno. That's beyond the scope of this change.