Add AtomLayoutStyle (#8465)

Extracts a shared struct with things that once might want to override
for any atomlayout-based widget.

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/egui_tests/tests/test_atoms.rs

@@ -139,6 +139,7 @@ fn test_atom_layout_nesting_and_direction() {
                 stack: ui.stack(),
                 style,
             })
+            .atom_layout
             .frame;
 
         let row = |direction: Direction| {