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>

.changeset/brave-foxes-dance.md

@@ -0,0 +1,22 @@
+---
+'@backstage/plugin-scaffolder-react': minor
+'@backstage/plugin-scaffolder': minor
+---
+
+Added experimental BUI (Backstage UI) form theme for scaffolder forms. All default field extensions render BUI variants when enabled.
+
+**Extension config:**
+
+```yaml
+app:
+  extensions:
+    - sub-page:scaffolder/templates:
+        config:
+          enableBackstageUi: true
+```
+
+**JSX props:**
+
+```tsx
+<ScaffolderPage formProps={{ EXPERIMENTAL_theme: 'bui' }} />
+```