Expose HasModuleInstancePerVU interface to external consumers

[simskij]

Expose HasModuleInstancePerVU to external consumers.

Feature Description

Currently, the HasModuleInstancePerVU interface is placed in an internal folder. This has the drawback that you can't check explicitly that an xk6 module implements it using the famous old var _ module.HasModuleInstancePerVU = new(Whatever) method. This might lead to extensions drifting away from the interface over time, without noticing, as this will only be checked at runtime.

https://github.com/loadimpact/k6/blob/67e33ab75d6a94659792ecf31afa7dd69e87c825/js/internal/modules/modules.go#L45-L50

Suggested Solution (optional)

While it is possible to redefine the interface in your xk6 package, or just skip the check and implement NewModuleInstancePerVU() anyway, it makes the xk6 extension unnecessarily fragile. Instead, I suggest we expose it by moving it outside the internal folder.

[na--]

It is still in internal because we don't have a firm agreement on how per-VU module instances should work. Please read through #1858 and comment if you have an opinion on the topic.