Add additional user options for Save Incremental feature

For starters two options could be simple to add:

1. Checkbox to enable separator "_" between original filename and increment.

2. Checkbox and amount of prefixed zeroes to add to the increment, such as 001 002. (for filename length consistency, in cases where external scripts process large amount of filenames is one of the rather niche usecases, but the cheap reason is that it just visually looks tidier in the file explorer)

By the way: Originally I had in mind a more powerful customization system to input custom suffix made by an regex definable increment and combining several tags (such as current system time-date, countless blender project properties / stats such as amount of scenes, amount of objects, but the ones that would make sense to have in a filename) would probably take a bit more effort and might be overkill in this area. I think it would also amount for a new separate Save command item altogether, but rather a better place where this kind of a filenaming system would make more sense is exporting, when you export lots of iterated scenes, renders, models, etc

Reference: projects.blender.org/blender/blender/commit/a58e5ccdec