🧹 Add basic unit tests (#148)

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.

__tests__/main.test.ts

@@ -1,3 +0,0 @@
-describe("TODO - Add a test suite", () => {
-  it("TODO - Add a test", async () => {});
-});