CLI support for multiple inputs

[Larandar]

This feature, in particular, is quite important to me: the ability to merge multiple documents in one query. It is a major feature of jq, and one missing from both xq and dasel

But it calls for a design choice:

1. Stay compatible with jq CLI and add --slurp-file and inputs, but quid of mixed format inputs?
2. Implement a xq way to do it (and possibly break xq specific options to be more streamlined?)