Async support in core

[43081j]

We should add async support to the core. This means chai-as-promised will no longer be needed in most cases (but will still be useful if you prefer the style it uses).

This kind of API:

expect(val).resolves.to.equal(303);

expect(val).rejects.to.throw("error");

This is very similar to vitest's existing chai extension. So it is also possible we can remove a lot of code from vitest once this becomes stable in core.

A few notes of things we'd need to do:

• All assertions would need to return a Promise<void> if the special flag is set
  • We can likely do this by wrapping all addMethod and addChainableMethod in something which conditionally awaits based on the special flag
• The special flag would be something like flag(this, 'promise') or flag(this, 'isAsync')
• A new resolves property would need to be added which sets the special flag
  • It would also need to set object to a new promise which, when resolved, sets object again to the resolved value
• A new rejects property would need to be added which sets the special flag

cc @koddsson @keithamus