[Feature] add preferred browser type to the device types

[mxschmitt]

It does mostly make not much sense to test Firefox on an iPhone since on iOS the browser engine is fixed. For Android its not but most of the users attend to use Chromium there.

By that it would be useful to attach the "preferred browser engine" to the browser descriptors. That would be probably:
a) For iOS devices WebKit
b) For Android devices Chromium

Once this feature is implemented we can then allow in the Playwright runner to run a test on Pixel 2 XL and iPhone 11 Pro Max which will then spin up a Chromium and WebKit in the correct resolutions / viewport. Without that change the user has to manually enter his preferred browser.

Reference:

• https://github.com/microsoft/playwright/blob/master/src/server/deviceDescriptors.ts