fix(forms): type NG_VALUE_ACCESSOR injection token as array (#29723)

NG_VALUE_ACCESSOR is a multi injection token, users can and
should expect more than one ControlValueAccessor to be
available (and this is how it is used in @angular/forms).

This is now reflected in the definition of the injection token
by typing it as an array of ControlValueAccessor. The motivating
reason is that using the programmatic Injector api will now
type Injector#get correspondingly.

fixes #29351

BREAKING CHANGES

NG_VALUE_ACCESSOR is now typed as a readonly array rather than
a mutable scalar. It is used as a multi injection token and as
such it should always be expected that more than one accessor
may be returned.

PR Close #29723

packages/forms/src/directives/control_value_accessor.ts

@@ -138,4 +138,5 @@ export interface ControlValueAccessor {
  *
  * @publicApi
  */
-export const NG_VALUE_ACCESSOR = new InjectionToken<ControlValueAccessor>('NgValueAccessor');
+export const NG_VALUE_ACCESSOR =
+    new InjectionToken<ReadonlyArray<ControlValueAccessor>>('NgValueAccessor');