Refactor group() and check() tight coupling with the VU.State

[na--]

This tight coupling is preventing us from implementing group() and check() purely in JavaScript. It is also preventing us from implementing similar functions, but with better UX (e.g. #1066, #2865) or that work better with async / await (#2848, #2728)

The tight coupling can be removed with only a very small loss of efficiency, by just having the MetricEngine process check and group_duration metrics as they appear and group them based on their group tag. Here is what basically needs to be removed:

k6/lib/vu_state.go

Lines 41 to 42 in 1d99b0b

	// Current group; all emitted metrics are tagged with this.
	Group *Group

k6/lib/runner.go

Lines 71 to 72 in 1d99b0b

	// Returns the default (root) Group.
	GetDefaultGroup() *Group

k6/js/modules/k6/k6.go

Lines 98 to 104 in 1d99b0b

	g, err := state.Group.Group(name)
	if err != nil {
	return goja.Undefined(), err
	}
	old := state.Group
	state.Group = g

k6/js/modules/k6/k6.go

Line 169 in 1d99b0b

check, err := state.Group.Check(name)

[olegbespalov]

For the record. After an internal discussion, we decided to clean a milestone since the issue was jumping between milestones without completion.

Once we determine which milestone it lands, we set the right one.