`browser.awaitNavigation()` for browser-initiated navigation

[sjelin]

Even with #3857, browser-initiated navigation is going to cause problems if/when a page unload interrupts a selenium command (example). I suggest we add a new function to help users to deal with this:

browser.awaitNavigation: (matcher: string | (url: string) => boolean) => wdpromise<void>

This function will behave basically like browser.get does after the location change.

[sjelin]

Blocked on #3857, since without the defer label will not get set