https://stackoverflow.com/questions/65567057/how-can-i-get-started-developing-extensions-for-python-idle prompted me to finish this enough to merge it.

My intention was that in the absence of a selection, all lines should be prefixed, not just the current one.  My impression is that some other formatters do this, but maybe not.  In any case, it does not matter exactly what this dummy example does, as long as it works as documented.