Feature Request: Support "export" package

[mikicho]

I would like to utilize the export assertion library without relying on jest as a test runner. Although it currently operates well at runtime, we need to address some typing issues. I have tackled this by modifying my global.d.ts file and it is functioning (almost) properly.

declare module 'expect' {
  interface Matchers<R> extends CustomMatchers<R> {}
  interface AsymmetricMatchers extends CustomMatchers<any> {}
  // expect.not... returns any due to https://github.com/microsoft/TypeScript/issues/50638 (Expect.Inverse<Omit<...>>)
}

But I'm not a Typescript expert so I'm not sure how to fix the expect.not scenario.