introducing the "convert" command

[pelikhan]

A very common scenario for AI is to munge through a file and rewrite it or write a result somewhere. To help streamline this scenario we now have the "convert" command (PR #953)

• apply spell check script in place

genaiscript convert spell-check **/*.md --rewrite

There is some smart inside the command to extrac the LLM output write the interresting part out. We could add more feature like a "appenfile" option and so forth. The goal of this command is to go from 1 file to N files quickly.

Unlike run, this command will do a full script run on each individual files.