feat(scaffolder): add BUI theme for scaffolder forms (#33053)

* feat(scaffolder): add BUI theme for scaffolder forms

Add a Backstage UI (BUI) form theme as an alternative to the Material
UI theme. Toggled via formProps.theme or enableBackstageUi page config.

Includes BUI widgets, templates, field extension variants, and a ported
React Aria Autocomplete component.

Signed-off-by: benjdlambert <ben@blam.sh>

* refactor(scaffolder): use BUI Combobox and CheckboxGroup for form widgets

Signed-off-by: benjdlambert <ben@blam.sh>

* chore(scaffolder): enable BUI form flag and add kitchen sink demo template

Signed-off-by: benjdlambert <ben@blam.sh>

* fix(scaffolder): use outlined input style for BUI form widgets

Signed-off-by: benjdlambert <ben@blam.sh>

* fix(scaffolder): address BUI form PR feedback

Signed-off-by: benjdlambert <ben@blam.sh>

* fix(scaffolder): format CSS and regen API reports

Signed-off-by: benjdlambert <ben@blam.sh>

---------

Signed-off-by: benjdlambert <ben@blam.sh>

app-config.yaml

@@ -70,6 +70,11 @@ app:
     - nav-item:scaffolder
     - nav-item:app-visualizer
 
+    # Opt in to the experimental BUI scaffolder form theme
+    - sub-page:scaffolder/templates:
+        config:
+          enableBackstageUi: true
+
     # Pages
     - page:catalog/entity:
         config: