add comparator $contains

[ludmillaZ]

need a contains for filter

[clue]

Thanks for your contribution, the code looks good to me and your addition is very much appreciated! 👍

However, the reason for not having this merged in yet is that this projects aims to be the reference implementation of the https://github.com/clue/json-query-language specification. The spec (draft) currently makes no mention of an $contains operator. I'd love to discuss this in the context of the specification, so a new issue over there would be very much appreciated.

I can certainly see where you're coming from and see some need for additional comparision operators. However, once we start adding a $contains operator, we'll likely see a need for similar operators like $startsWith and $endsWith as well. Also, the name $contains does not imply a data type, so is this limited to substring matching (perhaps we should consider a $regex operator?) or does this also apply to arrays?

[clue]

Also, the name $contains does not imply a data type […]

After some further consideration, this turns out to be a good thing. The future $contains operator should work on strings (substring matching), arrays (element search) and objects (key lookup).

The specification for the $contains operator is being worked on here: clue/json-query-language#9

Once then new spec has been released, we should update this PR.

[clue]

Just FYI: The spec has been updated and the new $contains operator has been merged. The spec has not been released yet.