refactor(forms): move CompatFieldState to be an interface

This is slightly more clear than using a declared `type`, and nicer for
documentation. Functionally it should not behave any differently.

Author: alxhub

goldens/public-api/forms/signals/index.api.md

@@ -65,9 +65,10 @@ export interface ChildFieldContext<TValue> extends RootFieldContext<TValue> {
 }
 
 // @public
-export type CompatFieldState<TControl extends AbstractControl, TKey extends string | number = string | number> = FieldState<TControl extends AbstractControl<unknown, infer TValue> ? TValue : never, TKey> & {
+export interface CompatFieldState<TControl extends AbstractControl, TKey extends string | number = string | number> extends FieldState<TControl extends AbstractControl<unknown, infer TValue> ? TValue : never, TKey> {
+    // (undocumented)
     control: Signal<TControl>;
-};
+}
 
 // @public
 export type CompatSchemaPath<TControl extends AbstractControl, TPathKind extends PathKind = PathKind.Root> = SchemaPath<TControl extends AbstractControl<unknown, infer TValue> ? TValue : never, SchemaPathRules.Unsupported, TPathKind> & {

packages/forms/signals/src/api/types.ts

@@ -319,12 +319,15 @@ export interface FieldState<TValue, TKey extends string | number = string | numb
  * @category interop
  * @experimental 21.0.0
  */
-export type CompatFieldState<
+export interface CompatFieldState<
   TControl extends AbstractControl,
   TKey extends string | number = string | number,
-> = FieldState<TControl extends AbstractControl<unknown, infer TValue> ? TValue : never, TKey> & {
+> extends FieldState<
+    TControl extends AbstractControl<unknown, infer TValue> ? TValue : never,
+    TKey
+  > {
   control: Signal<TControl>;
-};
+}
 
 /**
  * Allows declaring whether the Rules are supported for a given path.