Add the "@actions/exit" package

It is useful to have the exit logic separated into its own package

packages/exit/__tests__/exit.test.ts

@@ -0,0 +1,19 @@
+import * as exit from '../src/exit'
+
+it('exits successfully', () => {
+  jest.spyOn(process, 'exit').mockImplementation()
+  exit.success()
+  expect(process.exit).toHaveBeenCalledWith(0)
+})
+
+it('exits as a failure', () => {
+  jest.spyOn(process, 'exit').mockImplementation()
+  exit.failure()
+  expect(process.exit).toHaveBeenCalledWith(1)
+})
+
+it('exits neutrally', () => {
+  jest.spyOn(process, 'exit').mockImplementation()
+  exit.neutral()
+  expect(process.exit).toHaveBeenCalledWith(78)
+})