add a shortcut to count the number of entries/keys in list/dict

[teto]

just a feature I could have used a few minutes ago. I regenerated a json and noticed it was much smaller than before. Went from 1000 to 10 elements and I opened it with fx just to check if I could count the number of items in a list.
Since there are so few shortcuts for now, I thought it could be a fun nice addition. Otherwise feel free to close and thans for the beautiful software.

[antonmedv]

Nice idea! Also I'm thinking of adding ability to run commands in fx directly. Like len or .length.