Split internal.d.ts into multiple files

[tommy-mitchell]

Splits the types in internal.d.ts into multiple category-based files to improve maintainability, with the following structure:

\_ source/
   \_ internal/
      \_ array.d.ts
      \_ characters.d.ts
      \_ keys.d.ts
      \_ numeric.d.ts
      \_ object.d.ts
      \_ string.d.ts
      \_ tuple.d.ts
      \_ type.d.ts (misc types)
      \_ index.d.ts (so imports don't need updated)
   \_ ...
\_ test-d/
   \_ internal/
      \_ [category]/
         \_ [type].ts
   \_ ...

Other miscellaneous changes:

• Updates test script to detect source files in subfolders.
• Removes unused type imports.
• Renames ArrayMax/ArrayMin to TupleMax/TupleMin. This can be reverted if desired.

I tried maintaining the order of types and their formatting when copying them over, but I could've missed something.

[sindresorhus]

I don't mind this, but I also don't see a strong need to do it. Ideally, many of these types should be exported publicly instead (#676).

One thing I don't want is to nest the tests into folder. I think all the internal types should be top-level tests in the tests/internal folder.

[tommy-mitchell]

Updated tests and exposes some internal types that I found requests for in the issues. Please see the lines marked TODO for outstanding questions.

[sindresorhus]

When I mentioned exporting types, I didn't really mean in this pull request. It would be better to have separate pull requests for each exposed type, to make it easier to review and also to track changes for release notes.

[tommy-mitchell]

Unexposed types and added to separate PRs.

[tommy-mitchell]

Do we need both of these types?

[sindresorhus]

I don't know. I don't think that's relevant to this pull request though. Open a separate issue about it.