refactor: avoid prototype pollution in keyboard manager

Makes the check whether an input is a modifier more robust against prototype pollution.

(cherry picked from commit 68b3fd2)

Author: crisbeto

src/aria/private/behaviors/event-manager/keyboard-event-manager.ts

@@ -60,7 +60,7 @@ export class KeyboardEventManager<T extends KeyboardEvent> extends EventManager<
   }
 
   private _normalizeInputs(...args: unknown[]) {
-    const withModifiers = Array.isArray(args[0]) || (args[0] as string) in Modifier;
+    const withModifiers = Array.isArray(args[0]) || Modifier.hasOwnProperty(args[0] as string);
     const modifiers = withModifiers ? args[0] : Modifier.None;
     const key = withModifiers ? args[1] : args[0];
     const handler = withModifiers ? args[2] : args[1];