Add standalone mode for integration testing

[rg-wood]

Add a test mode for client integration testing. The package was modified to be publishable and installable:

npm i -D activitypub-starter-kit

The code was modified to be more modular so that it can be started programmatically without the need for environment variable or .env configuration. A utility method was added to quickly setup a test instance:

const app = ActivityPubApp.testApp();

app.start();

// ... tests

// cleanup
app.stop();

This will start a test instance on http://localhost:3000.

See here for an example test suite:

https://github.com/actionslaw/actionslaw-action/blob/4297ccaba1de48c1a638dc5fe0df3a40824931f6/test/integration/ActivityPub.test.ts