Add mockFn.mock.lastCall to easily retrieve the last call arguments

[deriegle]

Summary

When switching from using sinon mocks to jest, one of the nice things that sinon provided was the ability to retrieve the last call arguments easily. I know that you can use mockFn.mock.calls[mockFn.mock.calls.length - 1] to retrieve it, but mock.lastCall seems a lot easier when you're doing it in a bunch of places. I wanted to at least try adding it and see what the response was. I understand if this is not needed, but it seems like a nice little improvement.

Test plan

I've updated types, added tests for the feature and updated documentation. I ran yarn test for the entire test suite to verify that it was working. (Please let me know if there is additional testing that is necessary for this change.)

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[SimenB]

So sorry, this landed in #12285... Didn't know we already had a PR for it 😬

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