Docstring formatting: Preserve tab indentation when using indent-style=tabs

[MichaReiser]

Summary

This PR ensures that the formatter preserves tabs inside of docstring when using indent-style=tab.

Today, the formatter converts all docstring-indent to spaces, regardless of the configured indent-style.
This is unexpected for users and creates with E101 (mixed-spaces-and-tabs). For example:

def tab_argument(arg1: str) -> None:
	"""
	Arguments:
		arg1: super duper arg with 2 tabs in front
	"""

Gets reformatted to

def tab_argument(arg1: str) -> None:
	"""
	Arguments:
	        arg1: super duper arg with 2 tabs in front
	"""

Even though the user expliclty wants tab indentations.

The formatter won't automatically convert multiples of indent-width spaces to tabs because it can break
ASCII arts (and other formatting that use spaces for alignmen).

The downside of this change is tha the formatter won't enforce a consistent indentation inside of docstrings when using indent-style=tab.

Fixes #8430

This PR does not yet address that our formatter always expands tab to 8 spaces (instead of the configured indent-width)

It's not necessary to gate this change behind preview because it doesn't change already formatted code.

Test Plan

added tests, cargo test

[github-actions]

ruff-ecosystem results

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[BurntSushi]

This looks great to me! I think it might help to polish up the Indent type a touch, but I love the approach. :)