feat: add scroll duration

[SunGuoQiang123]

1. add scroll duration props
2. fix scrollTop compatible bugs: sometimes should get scrollTop by document.body.scrollTop and sometimes by document.documentElement.scrollTop
3. change toggle backTop btn from v-if to v-show, maybe this toggle may occur often

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[weituotian]

it is good!i agree

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.