@std/expect: export `getMockCalls` & `MockCall`

[GerkinDev]

Is your feature request related to a problem? Please describe.

I'm using @std/expect#fn to record calls. I want to be able to do complex assertions on calls to the function, that are not easy to do using expect(fn).toHaveBeenCalledWith

Describe the solution you'd like

Be able to get the mock calls records and manipulate them by myself. It would also help writing custom assertions on mock calls

Describe alternatives you've considered

Copy/paste the fn implementation and manage calls directly in my test code

[lionel-rowe]

It probably shouldn't be considered part of the public API as it's not in the TS types, but the calls are exposed via a non-unique symbol, so they can be accessed easily enough at runtime:

import { fn, expect } from '@std/expect'
const mockFn = fn((a, b) => a + b)
mockFn(1, 2)
expect(mockFn[Symbol.for('@MOCK')].calls[0].args).toStrictEqual([1, 2])

Given that, getMockCalls seems kinda redundant, it'd make more sense to re-export MOCK_SYMBOL and add & { [typeof MOCK_SYMBOL]: { calls: MockCall[] } } to the return type of fn.

[GerkinDev]

This would be more than enough to export the MOCK_SYMBOL and add the relevant type def. However, even if it isn't a unique symbol, doing Symbol.for('@MOCK') is not that user friendly and looks kind of like a magic string, and would probably require users to lookup the implementation of the mock fn to know this trick. Importing it looks better to me