I know this has been suggested before, along with many other features for the script editor including syntex checking, coloring, auto-completion, etc.
My response is that mIRC's script editor is exceedingly simple by design.
It's NOTEPAD simple.
I recommend that anyone interested in a better editor should go ahead and write an external editor. Using SendMessage(), it is entirely possible to seamlessly link the external editor to mIRC... with possible exception to the ALT+R accelerator.
With an external editor, features can be added and updated more frequently allowing Khaled to work on more important issues. It would also keep the mIRC executable to a minimum, as such features could potentially double mIRC's size.
As I understand it, free standing programs already offer many of these features and can be configured to understant mIRC syntex. I don't have these names off hand, but I'm sure they can be Searched for.