refactor: extract js and js/modulestest parts that are to do with setupping the js environment into a single place

[mstoykov]

Currenlty both the js package and js/modulestest do a number of things to configure Sobek.runtime and modules.ModuleSystem in order to have the same behaviour.

This still doesn't actually make it so and does require that a number of realistically internal details are exported but are in practice only meant to be used internally:

• the whole eventloop
• modules.ModuleSystem and co.

• k6/js/common/bridge.go

  Line 71 in ddc3b0b

  type FieldNameMapper struct{}

  - which maybe we should just remove ?
• https://github.com/grafana/k6/blob/master/js/common/randsource.go

Likely many more.

It also happens that we have two implementations that make it harder to work around.