preprocessed field type

so Lighttouch interfaces can:
```
when document viewed
if document model has preprocessed field
trigger model's / document's CRUD events (can be processed async, but before return)
```

- [Business Rules module's "Dependent fields" feature: overlap discussion [#2913096] | Drupal.org](https://www.drupal.org/project/conditional_fields/issues/2913096)
- [(rules-related) Issues for Computed Field | Drupal.org](https://www.drupal.org/project/issues/computed_field?text=rules&status=All)
- [model validations and update callbacks · Issue #93 · foundpatterns/lighttouch](https://github.com/foundpatterns/lighttouch/issues/93)
- ["calculated field" site:drupal.org - Google Search](https://www.google.com/search?q=%22calculated+field%22+site%3Adrupal.org&oq=%22calculated+field%22+site%3Adrupal.org&aqs=chrome..69i57.8148j1j1&client=ubuntu&sourceid=chrome&ie=UTF-8)
- [Calculate other calculated field [#2879241] | Drupal.org](https://www.drupal.org/project/entityreference_calc/issues/2879241)
- [Entity Reference Calc | Drupal.org](https://www.drupal.org/project/entityreference_calc)
- [Calculated field from another calculated field in the same content type. [#1880294] | Drupal.org](https://www.drupal.org/project/computed_field/issues/1880294)
- [Commerce pre-calculated price field | Drupal.org](https://www.drupal.org/project/commerce_pc_field)
- [Calculated field widget [#62623] | Drupal.org](https://www.drupal.org/project/cck/issues/62623)
- [preprocessed field active model calculated computed - Google Search](https://www.google.com/search?q=preprocessed+field+active+model+calculated+computed&oq=preprocessed+field+active+model+calculated+computed&aqs=chrome..69i57.12743j1j1&client=ubuntu&sourceid=chrome&ie=UTF-8)
- [ruby on rails - Should I save a Model calculation as an attribute? - Stack Overflow](https://stackoverflow.com/questions/28516257/should-i-save-a-model-calculation-as-an-attribute)
- [entities - How add markup and prefix / suffix for computed field - Drupal Answers](https://drupal.stackexchange.com/questions/102347/how-add-markup-and-prefix-suffix-for-computed-field)
- [Knockout : Pure computed observables](https://knockoutjs.com/documentation/computed-pure.html)
- [Knockout : Computed Observables](https://knockoutjs.com/documentation/computedObservables.html)