Add examples for scenario reuse

[na--]

It seems like with scenarios, people want to put different scenarios in different files and execute them separately, but also execute all of them at once. The requisite k6 issue for making this use case better is grafana/k6#1342, but even now, people can achieve a lot by just having a main file that imports and re-exports individual scenarios. Recent examples where this has popped up:

• https://community.k6.io/t/run-two-several-scenario-files-in-parallel-or-serially-using-single-file/1584
• Can I run two scripts at the same time by a command? k6#1918

[simskij]

Good idea! We need more content around test project organization in general as almost all examples we currently have are single-file ones.

[na--]

Here is another recent example of a similar question: https://community.k6.io/t/to-have-a-shared-project-for-a-company-with-multiple-teams/1653

And a more complicated use case where someone had to wrap handleSummary() of a generic script but leave everything else the same: grafana/k6#1946

This latter case is interesting because I had to drop down to ES5.1, since the way k6 implements the ES6 export and import prevented me from using them instead of the ES5.1 require() and exports. So this might be worth documenting instead.