Add test and basic implementation for formatter preview mode (#8044)

**Summary** Prepare for the black preview style becoming the black
stable style at the end of the year.

This adds a new test file to compare stable and preview on some relevant
preview options in black, and makes `format_dev` understand the black
preview flag. I've added poetry as a project that uses preview.

I've implemented one specific deviation (collapsing of stub
implementation in non-stub files) which showed up in poetry for testing.
This also improves poetry compatibility from 0.99891 to 0.99919.

Fixes #7440

New compatibility stats:
| project | similarity index | total files | changed files |

|----------------|------------------:|------------------:|------------------:|
| cpython | 0.75803 | 1799 | 1647 |
| django | 0.99983 | 2772 | 35 |
| home-assistant | 0.99953 | 10596 | 189 |
| poetry | 0.99919 | 317 | 12 |
| transformers | 0.99963 | 2657 | 332 |
| twine | 1.00000 | 33 | 0 |
| typeshed | 0.99978 | 3669 | 20 |
| warehouse | 0.99969 | 654 | 15 |
| zulip | 0.99970 | 1459 | 22 |

crates/ruff_python_formatter/src/statement/clause.rs

@@ -391,7 +391,9 @@ pub(crate) fn clause_body<'a>(
 
 impl Format<PyFormatContext<'_>> for FormatClauseBody<'_> {
     fn fmt(&self, f: &mut Formatter<PyFormatContext<'_>>) -> FormatResult<()> {
-        if f.options().source_type().is_stub()
+        // In stable, stubs are only collapsed in stub files, in preview this is consistently
+        // applied everywhere
+        if (f.options().source_type().is_stub() || f.options().preview().is_enabled())
             && contains_only_an_ellipsis(self.body, f.context().comments())
             && self.trailing_comments.is_empty()
         {