Helpers to streamline working with event loop

[inancgumus]

Introducing some helpers can make adding Async API supporting code and tests easier. Another reason for doing this change is the k6 promise API is still experimental, and its API can change (see this).

k6ext.Promise

This helper makes it easier to switch our sync API to async.

Before:

rt := b.vu.Runtime()
cb := b.vu.RegisterCallback()
p, resolve, reject := rt.NewPromise()

go func() {
	select {
	case <-b.browserProc.lostConnection:
		cb(func() error {
			resolve(true)
			return nil
		})
	case <-b.ctx.Done():
		cb(func() error {
			reject(fmt.Errorf("browser.on promise rejected: %w", b.ctx.Err()))
			return nil
		})
	}
}()

return p

After:

return k6ext.Promise(b.ctx, func() (interface{}, error) {
	select {
	case <-b.browserProc.lostConnection:
		// resolves the promise with the value true
		return true, nil
	case <-b.ctx.Done():
		// rejects the promise with the error with the given error
		return nil, fmt.Errorf("browser.on promise rejected: %w", b.ctx.Err())
	}
})

k6ext.AbortingPromise

There can also be an AbortingPromise that will allow aborting the event loop when an error occurs. It's not clear whether we're ever going to need this, then again, it doesn't hurt to support it.