Add support for $where in query

[DJMGE]

When I use $where in a find query, such as:

mydb["mycollection"].find({"$where": f"this.files.filter(f => f.path.endsWith('.txt')).length >= 3"})
I get the NotImplementedError exception to state that $where operator is not yet supported in mongomock

This would be nice to have in the next version