Modal 弹窗弹出时允许锁定背景滚动

[xaios]

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

Modal 过长时，会有两条滚动条产生，且在弹窗层滚动背景，体验上会比较差

What does the proposed API look like?

Modal 增加一个 prop，允许用户自定义设置背景锁定，如果默认是开启锁定的就更好了

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[github-actions]

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