feat(useElementVisibility): support watch the real element

[sxzz]

relate #2160

Description

Cannot get the real HTMLElement, we should use unrefElement.

Also, if element is changed, it should also be updated.

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
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[songjianet]

LGTM

[mymx2]

also remove (el as HTMLElement)
const rect = (el as HTMLElement).getBoundingClientRect()
to
const rect = el.getBoundingClientRect()

[mymx2]

and what if use watch instead of tryOnMounted. also Considering use v-if both on the Vue component instance and component.

const el = resolveUnref(element)
if (!el) {
  elementIsVisible.value = false
}

el instanceof Element

const el = unrefElement(element)
if (!el || !(el instanceof Element)) {
  elementIsVisible.value = false
}

[sxzz]

@mymx2 MaybeComputedElementRef it must be a Element, so I think we needn't have repeat judgement in runtime code.