Improve Expectation["equal"] typings

[Scyfren]

equal takes in unknown to allow for "negative" equals:

expect(1).never.to.equal(undefined)

but this means there is no strong typing for "positive" equals:

// stringUnion: "foo" | "bar"
expect(stringUnion).to.equal("abcde") // is allowed when it shouldn't be
// also no autocomplete when writing inside equal(...)

This MR fixes that:

// stringUnion: "foo" | "bar"
expect(stringUnion).to.equal("abcde") // type error
expect(stringUnion).to.equal("foo") // allowed
expect(stringUnion).never.to.equal("abcde") // allowed

expect(1).never.to.equal(undefined) // allowed
expect(1).never.never.to.equal(undefined) // type error
expect(1).never.never.to.equal(1) // allowed

[Scyfren]

There seems to be a bug with this fix but I'm not sure how to fix it:

expect(1).to.never.to.never.equal("abc"); // allowed when it shouldn't be
expect(1).to.never.never.equal("abc"); // type error as expected

[Scyfren]

Fixed the bug mentioned above.

expect(1).to.never.to.never.equal("abc"); // now has type error as expected
expect(1).to.never.never.equal("abc"); // type error as expected

[Scyfren]

Simplified the implementation as suggested by Dionysusnu in the discord server.
One issue is that using multiple nevers will not work as expected:

expect(1).to.equal("abc"); // type error as expected
expect(1).never.never.to.equal("abc") // allowed when it shouldn't be

but I don't see why anyone would want to use multiple nevers. The simpler fix is more readable and maintainable.