chore(api)!: Overhauling config and rendering types

[hairyhenderson]

Exporting config.Config and changing Renderer-related types as a result. A bunch of renames and shuffles. Everything should remain status quo from a CLI perspective, but this is one of the bigger breaking changes coming to v4.0 for programmatic users.

Fixes #2073

[hairyhenderson]

I need to decide whether I want the type in the root package and rename it to something other than Config so as not to conflict with the deprecated type, or go ahead and completely break things and remove RunTemplates and change the deprecated Config type to this new one instead.

I'm going to remove the deprecated RunTemplates and Config - they've been deprecated for 5 minor releases (since 3.7.0), more than 4 years... It's time. But, I'll do that in a separate PR for clarity.

[hairyhenderson]

#2095 has the removals, will rebase this after it merges