* fix the mess of shared/("global") and per-command environments
Figure out where to better put default function overrides etc.

Part of the idea of splitting them was to make `here` value local for
command and constant, while `my_pos` always return your current
position.  It makes sense only for long-running commands, but I'm
actually not sure how well those would work.

* add more shortcuts
- core.get_objects_inside_radius - make `nearby_objects` magic?

* do more to prevent possibilities for crashes
I think minetest-snippets does more interesting stuff - shamelessly
copy relevant parts from there.

* add proper REPL
Register on chat message and let executing multiple commands without
typing `/eval`.  Maybe via `/repl` command...
* add a "pager" for long output
To not spam the chat and prevent "Internal error: String too long"