engine: resources: Add some compile time checks for groupers

These can "break" silently and not autogroup if we change the resource
and it no longer fulfills the interface. Add this compile time check to
prevent that.

engine/resources/http_server_ui_input.go

@@ -57,6 +57,8 @@ func init() {
 	engine.RegisterResource(httpServerUIInputKind, func() engine.Res { return &HTTPServerUIInputRes{} })
 }
 
+var _ HTTPServerUIGroupableRes = &HTTPServerUIInputRes{} // compile time check
+
 // HTTPServerUIInputRes is a form element that exists within a http:server:ui
 // resource, which exists within an http server. The name is used as the unique
 // id of the field, unless the id field is specified, and in that case it is