Publish preview types for Ember

[chriskrycho]

The basic approach here will be:

• [FEATURE] Introduce preview types #20180: publish ambient types at types/preview, with:
  • an index.d.ts which imports ember and @ember/* module declarations
  • those declarations as declare module { ... } definitions, so that the root import "just works"
• document how to opt into the preview types: import 'ember-source/preview'; in:
  • a dedicated blog post after the release of 4.7 and 4.8 beta
  • the release post for 4.8 beta (which can link to the dedicated blog post for more details)
• add type tests infrastructure for supported TS versions #20185

With that foundation in place, we'll be able to iteratively work on publishing types generated from source (instead of ambient definitions) and once they're stable, flip over to publishing in types/stable and using the typesVersions hack (until we can dependably use an exports map) to define that as stable types.

[chriskrycho]

We've done the things! More to do, but progress! 🚀