* chore: replace `AnyDuringMigration` for field value functions
* chore: removed unnecessary `KeyboardEvent` comments and `AnyDuringMigration` casts
* chore: cleaned up `doValueUpdate_` and `doClassValidation_` in `Field` subclasses
* fix: updated `FieldValidator` to allow returning `undefined` and restructured `setValue`
* fix: implemented initial `core/field_checkbox.ts` feedback
* fix: updated `Field` to accept `U` and `undefined` and reverted subclass constructor handling of the input value
* fix: reverted `getVars` to returning `string[]` and added related comment
* chore: removed unnecessary comment
* fix: updated `processValidation_` to no longer allow returning `undefined`
* fix: removed `Un` type alias for `undefined`
* chore: removed unnecessary string cast in `core/field_colour.ts`
* fix: updated `doClassValidation_` not to expect `null` since it will never come up in `setValue`
* fix: updated `doClassValidation_` to only allow `undefined` when the new value exists
* Updated `FieldValidator` type to expect `newValue` to exist
* cleanup: updated `picker` from type `Element` to type `HTMLElement`
* fix: updated `doValueInvalid_` type info in `core/field_input.ts` to handle `string` and `undefined`
* fix: reverted `getValue` in `core/field_checkbox.ts` to previous logic
* fix: updated the `Field` constructor to allow `value` to be optional
* chore: consolidated `Validation` with `FieldValidator` and `doClassValidation_`
* fix: reverted generic param `U` while handling diverging user input is being discussed
* fix: updated `doClassValidation_` return comment to work for TSDoc
* fix: misc keyboard event function tweaks
* chore: add validator and field value types
* chore/clean up unused default and unnecessary field type setting
* chore: removed IRegisterableField and updated various Field instances
* fix: remove unused field image validator function
* chore: added pass-through constructor to field_textinput
