Various improvements

[alcaeus]

This PR adds a few improvements I've noticed while working on some pipelines:

• $mergeObjects is also available as an expression operator. I've followed the same strategy we previously used for $first
• The defaults argument for $zip should be optional
• $add and $subtract only implemented ResolvesToNumber. This made them unsuitable for use in places where an integer was required (in my case the n argument to $slice). I've changed the operators to resolve to all numeric types individually. Note that the pipeline may error on the server if $add evaluates to a value of the wrong type, but we can't solve that without adding generics
• The $expr query operator was marked as a fieldQuery when it is actually a query similar to $and and $or.

[GromNaN]

It would be great to add tests on this operators to bury these changes.

[alcaeus]

Fixed another issue with $zip (the useLongestLength argument should be optional) and added tests from the docs for all updated operators

[GromNaN]

You even took the comments 👏🏻