Use tt extension for form scaffold template (#29676)

lib/templates/haml/scaffold/_form.html.haml.tt

@@ -0,0 +1,10 @@
+= simple_form_for(@<%= singular_table_name %>) do |f|
+  = f.error_notification
+
+  .form-inputs
+  <%- attributes.each do |attribute| -%>
+    = f.<%= attribute.reference? ? :association : :input %> :<%= attribute.name %>
+  <%- end -%>
+
+  .form-actions
+    = f.button :submit