More granular versioning for JavaScript API

[codebien]

Explicit when a JavaScript API has been added. It helps users to know what is the minimum k6 version they have to use.

It's a practice already used for other APIs' documentations, some examples:

1. Go
   

2. MDN
   

[oleiade]

For your information @heitortsergent, this is something we would aim to establish as a documentation item and practice for k6 v1.