BrowserType will no longer be an exposed type through Goja layer,
instead we will handle its functionality from our implementation in the
Go layer.

Adds a new browser registry to be held at the root module level, which
lifecycle spans throughout the whole test run. The registry will store
browser instances per iteration so we can handle their lifecycle from Go
layer based on the iteration lifecycle, which is tight to the VU
context.

Retrieves the scenario name for the given VU context.
This is required in order to build the key for the browsers pool, as the
VU iteration is just a monotonic integer that is shared per scenario.
Therefore, if we don't want to share the browser instance per scenario,
we have to build the pooll key based on the scenario name also.

Because browser is not controlled by explicit JS test code, now every
browser level method will automatically initialize the browser for the
iteration in case it has not been initialized yet in a previous call.
Once the browser is initialized it's stored in the browser registry
indexed by the iteration ID (composed by VUID-scenario-iterationID).
When the iteration ends, signaled by the VU context being done, the
browser is automatically closed and removed from the registry.

Because we want to handle browser lifecycle automatically from our Go
implementation, we can not build the browser context based on the VU
context, which will be closed at the end of every iteration. That would
result in a race condition between iteration and browser closing
procedure. Instead we will wait for the VU context to be closed and then
from our implementation explicitely call Browser.Close() method.

Previously, the browser implementation components, such as browser,
page, frame... were holding a context inherited from the vu.context
because the browser lifecycle was tight to it. Therefore, we were
passing these contexts when calling k6metrics.PushIfNotDone, as if that
context was done, it meant that the VU context was also done and at the
same time the VU metrics samples channel was closed. With the current
implementation, because the browser, page, frame... contexts do not
inherit from the VU.context, we can not use these contexts when calling
k6metrics.PushIfNotDone, as it is not a guarantee that the vu samples
channel is open, so we should use the VU context directly.