feat(theme): allow providing custom toggle-appearance function

[brc-dd]

Demo: https://64e7754fb34be10008d647e3--vitepress-docs.netlify.app/ (won't be there in docs)

closes #2347

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PS: if you want to change component appearance conditionally, do something like this:

const enableTransition = ref(false)

onMounted(() => {
  enableTransition.value = 'startViewTransition' in document && window.matchMedia('(prefers-reduced-motion: no-preference)').matches
})

<DefaultTheme.Layout :class="{ 'enable-transition': enableTransition }" />

.enable-transition .VPSwitchAppearance        { width: 22px !important; }
.enable-transition .VPSwitchAppearance .check { transform: none !important; }

But note that this will cause layout shifts and is not recommended.