Add type generic to Config

[mfix-stripe]

@segphault I'm not sure if we want to convolute our types in this way just to support Next.js (I'm actually leaning towards no…), but wanted to throw up this PR for consideration to solve https://github.com/markdoc/markdoc/issues/477

If you don't think we should go this direction, can you comment in the original thread and close it?

Closes https://github.com/markdoc/markdoc/issues/477