Expose @types/_internal

[nickmessing]

Is your feature request related to a problem? Please describe.
I'm working on vue-tinybase (really early-stage for now), and I use types defined as "internal" for some type computations. Is it an option to expose internal types too?

Describe the solution you'd like
Add "_internal" as an exported path to package.json

Describe alternatives you've considered
I could import using absolute path and make esbuild "bundle" that types, feels very hacky tho.
I am copy/pasting the "internal" types into the source code to be able to migrate to v5 but that doesn't feel very good either.

Additional context

[jamesgpearce]

I think this should be OK. Type system is pretty large and a few extra complexities on the edges won't hurt.

[jamesgpearce]

I look at this and some parts of it are pretty large to expose and document. Are there any key types you would really want to prioritize? Most easily I can just promote them up into the main packages.