This helps us by clarifying that this is a background context, and not
controlled by k6 (i.e. the VU context). The background context is to
only be used to control the lifecycle of chromium/connection. In later
commits vu context will be reintroduced to control the iteration
lifecycle so that k6 can abort the iteration and the browser module
doesn't block that from happening.

The background context will be used for the chromium subprocess,
connection and things that need to be controlled by the iterStart and
iterEnd (k6 event system) events. The k6Ctx is the context that is
controlled by the vu context. This will allow the iteration to abort
due to a SIGTERM while also allowing chromium to exit with endIter/exit
events.

At the moment this only introduces the k6Ctx, but it is still the
background context.

vuCtx is a clearer name which represents the VU context.

Now that Browser is setup to work with the different contexts, let's
actually set them up correctly.

This rename makes it clearer which context will be cancelled if the
function is called.

The background context isn't needed here. We're removing it to make
things clearer later on when we get to creating the Browser type, which
uses the cu context.

It didn't feel right having a goroutine where there was no way of
shutting it down. Now when the browser.Close() method is called it will
shut the init goroutine down too.

This is to ensure that we remember why we're using context.Background
for launching or connecting to chromium.

Encapsulate the retrieval of the browser count from the registry.

Encapsulate the retrieval of the iteration trace count from the
registry.

To be more idiomatic go