Add `uvu/spy` package

Hello @lukeed, I’m solving #9 issue, and when doing the tests I discover that your assert library doesn’t have a way to test if some function was called. In other libraries like jest we can do it by creating some reference to a mock function (jest.fn()for example) and after that, check if this function was called. What do you think about this feature?

[therealparmesh]

I too was looking for a small alternative to Sinon or Jest for spy functionality to pair with Uvu.

Might I suggest https://github.com/therealparmesh/snoop until Uvu reaches this point in the roadmap?

I’m going to publish the first version pretty soon and would greatly appreciate any guidance from the community.

[lukeed]

Looks great @therealparmesh! Awesome alternative (and name 😆)!

I think uvu/spy will still exist at some point in the future, but this is definitely a viable option.