indvidual -> individual

* Use consistent indentation style in YAML examples
* Fix typo 'indvidual'

- Document the release process in `CONTRIBUTING.md`
- Add development instructions to `CONTRIBUTING.md` (adapted from [download-artifact](https://github.com/actions/download-artifact))
- Add a test badge to the `README`
- Link to `CONTRIBUTING.md` in `README`
- Unify copy between `README.md` and `action.yml`
- Copy-pasta the CoC from [download-artifact](https://github.com/actions/download-artifact)

This is not intended to be a comprehensive test suite; I'm just trying to get us started by adding tests for some of the most important functionality.

This necessitated some minor refactoring. Previously, `main.ts` just directly invoked the main entrypoint function `run()`, which made it impossible to unit test the module without causing the side-effects of `run`. 

As a workaround I created  a new module `labeler.ts`, which just exports the functions we want to test without executing the main entrypoint, and simplified `main.ts` so that it only imports the entrypoint and executes it.

It's basically just a re-org. The diff makes it look way more complicated than it is.

Fix typo in README.md

Fix recursive globbing in our npm `format` script command and run all ts files through prettier 💅

Gotta love those trailing commas!

Update prettier to the latest version and re-prettify everything