Build: Allow creating internal templates that extend others and pass main.js options

[tmeasday]

This PR introduces the following changes:

1. The ability to add custom mainJs in repro-templates
2. Repro templates are separated in two sections: baseTemplates and internalTemplates.
   Internal templates reuse existing templates from baseTemplates, and add extra functionality on top, such as, having a custom mainJs with storyStoreV7: false so we can test storyStoreV6 configurations. Those internal templates have to have an id that starts with internal/ so they are grouped in the repro-sandboxes repo
3. Some QoL improvements for the repro command

The idea is to allow us to test more specific configurations, such as storyStoreV6.

This is still a WIP (types are broken the files I modified).

This PR allows two things in repro templates:

1. A template.mainConfig field that gets written to the main.js (this is a pretty simple change).
2. The ability for one template to extend another.

The second part is needed for the first, otherwise we'll end up creating duplicate repros for templates that differ only in configs. (Possibly we should just live with that in the short term?!?)