which editor does that? space width is defined by the font, and normally you use monospace fonts. i am unfamiliar with any editor that allows you to choose a different font for spaces. besides, it would only work for indents, it would mess up any alignment of code into columns.
None, but it's clearly possible to add since such handling is already present for tabs in almost any editor. I merely mean that there's no technical restriction.
But this option already exists for tabs that are pretty much built for this purpose. It's like adding a fifth wheel to a car because there's no technical restriction even though 4 work fine
