Add a linter/formatter

[Ethan-Arrowood]

Let's get some automated consistency from the beginning. I'd like to add a markdown linter that has a reasonable set of rules. I don't want to bikeshed config options so preference for something equivalent to Prettier that makes the decision for us.

The linter/formatter should run on commit (lets use husky + lint-staged).

There should be a GitHub Action that runs the linter/formatter and blocks the PR from being merged until it is passing.

[ljharb]

Please do not use husky; git hooks are bypassable, slow, and tools like husky can cause security issues by installing the hooks in the wrong repo. Markdown linting can be configured through eslint, and then preexisting editor integrations can handle it.