docs: improve scaladocs for TastyFormat
docs: improve scaladocs for TastyFormat

The main motivation behind this is to improve the reader experience when
looking at the TastyFormat docs on
https://scala-lang.org/api/3.x/dotty/tools/tasty/TastyFormat$.html since
in its current format all spacing is lost. This instead utilizes code
blocks in a valid scaladoc to ensure the spacing is kept. You can see
what this should look like by looking at
https://scala-lang.org/api/3.x/scala/quoted/Quotes$reflectModule.html
which uses the same approach.

Addresses https://github.com/lampepfl/dotty-feature-requests/issues/273

dotty-staging/dotty,lampepfl/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty