Declare enableFeature (#29927)

* Declare `enableFeature`

* Add missing newline

Co-Authored-By: zeppelin <gabor.babicz@gmail.com>

types/ember-feature-flags/ember-feature-flags-tests.ts

@@ -1,5 +1,6 @@
 import Features from 'ember-feature-flags';
 import 'ember-feature-flags/tests/helpers/with-feature';
+import 'ember-feature-flags/test-support';
 
 /** Static assertion that `value` has type `T` */
 // Disable tslint here b/c the generic is used to let us do a type coercion and
@@ -24,4 +25,5 @@ const setup = {
 };
 features.setup(setup); // $ExpectType void
 withFeature('new-homepage'); // $ExpectType void
+enableFeature('new-homepage'); // $ExpectType void
 assertType<boolean>(features.get('someFeature'));

types/ember-feature-flags/test-support/index.d.ts

@@ -0,0 +1 @@
+declare function enableFeature(name: string): void;

types/ember-feature-flags/tsconfig.json

@@ -47,6 +47,7 @@
     "files": [
         "index.d.ts",
         "tests/helpers/with-feature.d.ts",
+        "test-support/index.d.ts",
         "ember-feature-flags-tests.ts"
     ]
 }