expect `toBeNullish`

[MetRonnie]

Clear and concise description of the problem

Sometimes we don't care whether a value is undefined or null, we just want to test that it is nullish.

Suggested solution

I am proposing a new assertion, toBeNullish(). This would test value == null, i.e. that the value is either null or undefined.

Alternative

No response

Additional context

No response

Validations

• [x] Follow our Code of Conduct
• [x] Read the Contributing Guidelines.
• [x] Read the docs.
• [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.