Use EventTarget in worker / browser runtimes

[anniel-stripe]

Summary

1. Refactor _emitter property on the Stripe object to use EventTarget instead of EventEmitter in worker environments.
2. Add a base class PlatformFunctions that is implemented by WebPlatformFunctions and NodePlatformFunctions
3. Consolidate checking for streaming data and buffering logic into a tryBufferData platform function, which throws an error in workers if a ReadableStream is detected.

Workarounds because of Node <15

Event and EventTarget are only available in the global scope starting with Node 14. Since _StripeEvent needs to extend Event (to add the data object used by listener functions), I ended up creating a base class for PlatformFunctions and a separate WebPlatformFunctions that is the only place where StripeEmitter is imported.

In the tests I added to PlatformFunctions.spec.ts, I also added a check for the Node version so that WebPlatformFunctions test do not run in Node <= 14.

Testing

This gets rid of the last of the build errors when importing stripe-node in Cloudflare Workers without polyfills. I tested this new implementation in Cloudflare Workers / Pages and both emit events as expected.

[pakrym-stripe]

1. Let's refactor platform functions a bit so we don't have to push more logic into entry point.
2. Let's add some tests to confirm parity between event emitter implementations.
3. Let's decide what we want to do with file streams in non-node environments.

[pakrym-stripe]

few more nits