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>

yarn.lock

@@ -6994,11 +6994,13 @@ __metadata:
     "@backstage/test-utils": "workspace:^"
     "@backstage/theme": "workspace:^"
     "@backstage/types": "workspace:^"
+    "@backstage/ui": "workspace:^"
     "@backstage/version-bridge": "workspace:^"
     "@material-ui/core": "npm:^4.12.2"
     "@material-ui/icons": "npm:^4.9.1"
     "@material-ui/lab": "npm:4.0.0-alpha.61"
     "@react-hookz/web": "npm:^24.0.0"
+    "@remixicon/react": "npm:^4.6.0"
     "@rjsf/core": "npm:5.24.13"
     "@rjsf/material-ui": "npm:5.24.13"
     "@rjsf/utils": "npm:5.24.13"
@@ -7022,6 +7024,7 @@ __metadata:
     luxon: "npm:^3.0.0"
     qs: "npm:^6.9.4"
     react: "npm:^18.0.2"
+    react-aria-components: "npm:^1.14.0"
     react-dom: "npm:^18.0.2"
     react-router-dom: "npm:^6.30.2"
     react-use: "npm:^17.2.4"
@@ -7107,6 +7110,7 @@ __metadata:
     luxon: "npm:^3.0.0"
     qs: "npm:^6.9.4"
     react: "npm:^18.0.2"
+    react-aria-components: "npm:^1.14.0"
     react-dom: "npm:^18.0.2"
     react-resizable: "npm:^3.0.5"
     react-resizable-panels: "npm:^3.0.4"
@@ -41522,7 +41526,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"react-aria-components@npm:~1.17.0":
+"react-aria-components@npm:^1.14.0, react-aria-components@npm:~1.17.0":
   version: 1.17.0
   resolution: "react-aria-components@npm:1.17.0"
   dependencies: