Make k6 browser opt-out instead of opt-in

[ka3de]

Currently, since the inclusion of k6 browser as experimental module in k6 (v0.43.0), browser tests require and explicit environment variable to be set (specifically K6_BROWSER_ENABLED=true), making k6 browser module disabled by default and instead being an "opt-in" feature.

This issue proposes reverting that and instead make k6 browser enabled by default, but allow users to explicitly opt-out.
This will make browser tests more user friendly and still maintain "security capabilities" by supporting disabling any browser related functionality.

[ka3de]

This is N/A by now as browser option in scenario parameters definition is an explicit requirement.