model.ts is currently extremely large. This is the first step in an attempt to refactor it to be more easily navigable and reviewable. This commit breaks up `model.ts` into the following new files:
* `model/abstract_model.ts`: The remainder of the model, including the `AbstractControl` base class and helper functions which are used throughout.
* `model/form_control.ts`: `FormControl`, `FormControlOptions`, and helpers, plus the constructor and untyped friends.
* `model/form_array.ts`: `FormArray` and untyped friends.
* `model/form_group.ts`: `FormGroup` and untyped friends.
This first phase is a purely mechanical code move. There is no new code at all, and no interfaces have been separated.
PR Close#45217
