Formatter should clean up leading white space in front of an indented raw string line

Inspired by https://github.com/dotnet/sdk/pull/55542#discussion_r3837081177

Code example:

Pay attention to the fact that between `Line 1` and `Line 3` there is an empty line with just white spaces. Since the raw string is indented the chunk of white space on that line is effectively useless. Formatter should clean up things like this

No source file or test is named. Start by reproducing the raw-string formatter example and tracing the formatter's handling of indented raw-string lines; done means whitespace-only lines inside the raw string are cleaned up without changing meaningful content.

Written by the indexing model from the issue text.