refactor(i18n): I18nPipe uses NgLocalization (#9313)

and some refactoring

modules/@angular/common/src/localization.ts

@@ -0,0 +1,18 @@
+/**
+ * @experimental
+ */
+export abstract class NgLocalization { abstract getPluralCategory(value: any): string; }
+
+/**
+ * Returns the plural category for a given value.
+ * - "=value" when the case exists,
+ * - the plural category otherwise
+ *
+ * @internal
+ */
+export function getPluralCategory(
+    value: number, cases: string[], ngLocalization: NgLocalization): string {
+  const nbCase = `=${value}`;
+
+  return cases.indexOf(nbCase) > -1 ? nbCase : ngLocalization.getPluralCategory(value);
+}