Support `$position` array update operator's modifier

[AlekSi]

Depends on #828

https://www.mongodb.com/docs/manual/reference/operator/update/position/

DoD:

• [ ] valid cases (for $push with $each, different positions)
• [ ] invalid case (e.g. attempt to use without $each)
• [ ] test case for negative index
• [ ] test case for positive index