fix: record emitted events once when a devtools hook is installed - #2965

When __VUE_DEVTOOLS_GLOBAL_HOOK__ exists, every mount wraps hook.emit once more (#2311), so after N mounts a single $emit is recorded N times. And if the hook is replaced after Vue's renderer was created (Vue keeps the old one), emitted() is empty.

vi.stubGlobal('__VUE_DEVTOOLS_GLOBAL_HOOK__', { emit: vi.fn() })
mount(Comp)
const wrapper = mount(Comp)
wrapper.vm.$emit('foo')
wrapper.emitted('foo') // [[], []]

This always installs the VTU hook again, as before #2311, and forwards every event (and cleanupBuffer) to the current global hook, so devtools still receive everything (#2310). I checked this with @vue/devtools-kit: app:init, component:* and perf:* each arrive once.

Seen in ComfyUI_frontend, which stubs the hook in a beforeEach: emitted() counts depend on how many mounts ran before.

Repro on 2.5.1: playground

cc @webfansplz