Standardize formatting with a linter and add a lint check to CI

[schneems]

• Add https://github.com/testdouble/standard as a linter to the project.
• Fix lint errors
• Add a check to CI to ensure PRs are correctly formatted

Help wanted

Also, I'm looking for help maintaining this project in the future. I ask anyone interested to please send a PR and mention that you're interested in helping. You can email me if you've previously had a commit merged in this project.

A maintainer will gain commit access and deploy access on rubygems if they don't have it already. A maintainer will be expected to:

• Update CI to keep tests passing and test new Ruby releases
• Review, merge, or close PRs
• Read and respond to issues
• Any feature or bugfix work that you desire