fix(forms): Allow NonNullableFormBuilder to be injected.

Based on early feedback, calling `fb.nonNullable.group(...)` continues to be clunky for a form with many such groups. Allowing `NonNullableFormBuilder` to be directly injected enables the following:

```
constructor(private fb: NonNullableFormBuilder) {}
```

goldens/public-api/forms/index.md

@@ -714,6 +714,9 @@ export interface NonNullableFormBuilder {
     }>;
 }
 
+// @public (undocumented)
+export const NonNullableFormBuilder: InjectionToken<NonNullableFormBuilder>;
+
 // @public
 export class NumberValueAccessor extends BuiltInControlValueAccessor implements ControlValueAccessor {
     registerOnChange(fn: (_: number | null) => void): void;

packages/forms/src/form_builder.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Injectable} from '@angular/core';
+import {inject, Injectable, InjectionToken} from '@angular/core';
 
 import {AsyncValidatorFn, ValidatorFn} from './directives/validators';
 import {ReactiveFormsModule} from './form_providers';
@@ -345,6 +345,9 @@ export interface NonNullableFormBuilder {
       asyncValidator?: AsyncValidatorFn|AsyncValidatorFn[]|null): FormControl<T>;
 }
 
+export const NonNullableFormBuilder = new InjectionToken(
+    'NonNullableFormBuilder', {providedIn: 'root', factory: () => inject(FormBuilder).nonNullable});
+
 /**
  * UntypedFormBuilder is the same as @see FormBuilder, but it provides untyped controls.
  */