🧹 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__/labeler.test.ts

@@ -0,0 +1,29 @@
+import { checkGlobs } from '../src/labeler'
+
+import * as core from "@actions/core";
+
+jest.mock("@actions/core");
+
+beforeAll(() => {
+  jest.spyOn(core, "getInput").mockImplementation((name, options) => {
+    return jest.requireActual("@actions/core").getInput(name, options);
+  });
+});
+
+const matchConfig = [{ any: ["*.txt"] }];
+
+describe('checkGlobs', () => {
+  it('returns true when our pattern does match changed files', () => {
+    const changedFiles = ["foo.txt", "bar.txt"];
+    const result = checkGlobs(changedFiles, matchConfig);
+
+    expect(result).toBeTruthy();
+  });
+
+  it('returns false when our pattern does not match changed files', () => {
+    const changedFiles = ["foo.docx"];
+    const result = checkGlobs(changedFiles, matchConfig);
+
+    expect(result).toBeFalsy();
+  });
+});