fix(vue): eslint/tsconfig migration + resolve type errors

@robonen/vue (toolkit): migrate to eslint flat config + composite tsconfig;
fix composable + test type errors (writable computed returns, null guards,
overload-compatible signatures, typed test helpers) — all type-level.

vue/toolkit/src/composables/browser/useIntervalFn/index.test.ts

@@ -1,4 +1,4 @@
-import { describe, expect, it, vi, beforeEach, afterEach } from 'vitest';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { defineComponent, effectScope, nextTick, ref } from 'vue';
 import { mount } from '@vue/test-utils';
 import { useIntervalFn } from '.';