* implement UI for the formula field

* consider utilizing the components for generated work package subjects

* [pattern input](https://github.com/opf/openproject/blob/dev/app/components/work_packages/types/pattern_input.rb)

* [pattern input controller](https://github.com/opf/openproject/blob/dev/frontend/src/stimulus/controllers/pattern-input.controller.ts)

* offer dropdown containing

* mathematical operators and parenthesis Begin of the insertion

* note that mathematical operators should _not_ be tokenized (rendered as bubble) after selection from the dropdown.

End of the insertion Begin of the deletion

End of the deletion * custom fields of type integer and float

* custom fields of type calculated value

* consider custom fields when validating the formula