Add action to run tests on PRs

[ezmiller]

Goal

Add some automated tests and linting to be run on pull requests. Why? This can help build a common reference point about code failures and code style.

Implementation

This PR uses github actions that makes running such tests / linters quite easy. I added two different actions. One for linting and one for tests.

The linting action runs two linters

• cljfmt - focuses on code formatting things like indentation. It can also automatically fix problems when run with lein cljfmt fix (as opposed to lein cljfmt check).
• eastwood - is a linter that checks more broadly for things like missing vars, etc.

We can always modify and customize these linting tools as we see fit, but they'll help us converge on a common style that also reflects the community's preferences.

How does it work?

Basically when you open a PR the tests / and linters will run. They show up at the bottom of the PR page (i.e. this type of page), and look like this:

You can look at the logs of these actions, i.e. this is the last run of the tests on this PR.

[ezmiller]

@rsiddharthan thank you for the review!