Add assertions API and improved functional testing capabilities to k6

[oleiade]

What

We have led an internal effort to provide a native assertions API to k6 as well as various improvements in order to make k6 better suited to functional testing: synthetic monitoring, browser testing, smoke testing, etc.

To track this work, we have created a couple of issues tracking the different key changes that we think need to happen for k6 to reach this objective.

Definition of done

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