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>

plugins/scaffolder/package.json

@@ -98,6 +98,7 @@
     "lodash": "^4.17.21",
     "luxon": "^3.0.0",
     "qs": "^6.9.4",
+    "react-aria-components": "^1.14.0",
     "react-resizable": "^3.0.5",
     "react-resizable-panels": "^3.0.4",
     "react-use": "^17.2.4",