chore: use standard

[mperham]

Integrate the Standard linter into the build.

I've included both safe and unsafe automated fixes but I did not make any manual code changes. Please run bundle exec rake standard to see the list of lint warnings.

I also included a .standard.yml with ruby 2.3.0 as the minimum version. If you bump this, it will use newer language features. Unfortunately my local rubies are mostly horked so I could only test Ruby 3.3.