Replace usage of "browsing contexts" in favor of "navigable"

[domenic]

In whatwg/html#6315, HTML created an new concept, "navigable", which is almost certainly the correct thing for this specification to target, instead of browsing contexts.

In particular, a navigable stays the same even across browsing context swaps such as those induced by cross-origin opener policy. So for example, all the navigation-related stuff in WebDriver was previously broken in COOP cases, as the browsing context would be left untouched by the navigation and a new second browsing context, which WebDriver wasn't tracking, would be created. Now, by having WebDriver interface with the navigable, this problem can be avoided.

This will probably need some pretty extensive surgery. I am happy to help advise as needed.

/cc @marcoscaceres

[marcoscaceres]

Tiny start to addressing some of this:
#1694

[whimboo]

Please also see w3c/webdriver-bidi#91 which is for WebDriver BiDi.