Add support for tabs as indent

The whole tabs vs. spaces debate is exhausting. I use tabs, and lots of other people use tabs. There's really strong arguments for preferring tabs. It would be nice if this tool supported tabs, too.

Since there is already a mechanism in place to switch between 2 and 4 spaces, adding tab support should not be wildly challenging. If you are open to supporting tabs, I might even be able to contribute it (I added tab support to ktlint).

Just think: if you _only_ supported tabs, users could pick 2 or 4 character rendering (or any other value) and you wouldn't have needed an option at all!

Start by locating the existing mechanism that switches between 2- and 4-space indentation and trace how indentation is applied throughout the formatter. Determine the scope of tab support and verify that users can format Kotlin with tabs and choose the rendered tab width.

Written by the indexing model from the issue text.