Improve typings

[kanongil]

This PR includes 1 patch to loosen types to fix #162, 1 to add a missing NaN() method, and 2 patches that tightens the types.

The 24de1bc commit is the most invasive, and splits type specific methods off, so that they won't appear unless they can be used on the expected type. Eg. these will now fail the type check:

expect(true).to.startWith('t');
expect('123').to.be.greaterThan(10);
await expect(() => 'hi').to.reject();

All of these patches should probably be considered bug fixes.

[kanongil]

Ping?

[Nargonath]

Per your comment

All of these patches should probably be considered bug fixes.

I added the bug label. This is to set the scene for timetrack.

[devinivy]

This is now released in 8.0.4 👍