Add support for `$regexFind`, `$regexFindAll`, and `$regexMatch` aggregation operators

[christophstrobl]

Available since MongoDB 4.2.

• https://docs.mongodb.com/manual/reference/operator/aggregation/regexFind/
• https://docs.mongodb.com/manual/reference/operator/aggregation/regexFindAll/
• https://docs.mongodb.com/manual/reference/operator/aggregation/regexMatch/