[PoC] Parse exported script options with lib.TestPreInitState access

[na--]

This is built on top of #2999 and is a PoC of how we can parse exported script options while we have access to the *lib.TestPreInitState.

Because of the changes in #2999, JavaScript modules also have access to the *lib.TestPreInitState, so it can be used as a central place to coordinate things. For example, JS modules could presumably "register" scenario options specific to them, which could then be used when parsing and validating the options.

This is not intended for merging as it is, even though it slightly improves the UX (multiple errors when parsing the script options can now be returned at the same time), it's just a PoC that could be used for discussion. There are plenty of unresolved issues with it! For example, it doesn't handle .json config files or options that are in a .tar archive's metadata.json file. Both of these could presumably be made to work in a similar way, since we have the *lib.TestPreInitState in both cases, but it would require further refactoring.

The important part of this PoC is being able to statefully parse JSON files, having access to some k6-specific context while we do it, without fully rewriting the whole config. That said, the complexity overheads are somewhat big, so I am not sure if this is the way we want to go ahead in resolving #883, probably not... 😞

[codecov-commenter]

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