$EDITOR for SQL/PRQL commands

[Mr-Andersen]

Is your feature request related to a problem? Please describe.

My PRQL expression got really big. It would be much nicer to write it in a more capable editor. Also, that would make it easier to save the expression elsewhere --- currently, this is tedious process of copying the expression in chunks.

Describe the solution you'd like
A shortcut to open current expression in $EDITOR? E.g. I press ;, then I press Ctrl+; (or whatever), and $EDITOR replaces lnav. On exit, whatever I've left in editor is used as new expression.

[tstack]

See #1380

Right now, just the :comment command is wired up to use the multi-line prompt. I'll be doing the same for SQL/PRQL soon.