refactor(frontend): use Symbol for vue provide/inject

packages/frontend/src/di.ts

@@ -0,0 +1,12 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type { InjectionKey, Ref } from 'vue';
+import type { IRouter } from '@/nirax.js';
+
+export const DI = {
+	routerCurrentDepth: Symbol() as InjectionKey<number>,
+	router: Symbol() as InjectionKey<IRouter>,
+};