index-list height prop should be string

`index-list` 组件，这个 `height` 属性应该为 string，因为我外部传入的时候，传值可能为 `auto`
https://github.com/ElemeFE/mint-ui/blob/master/packages/index-list/src/index-list.vue#L85

Open packages/index-list/src/index-list.vue at the referenced line and inspect how the index-list height prop is declared. Confirm the prop accepts the string value "auto", update its declared type accordingly, and verify the component still handles the existing height usage.

Written by the indexing model from the issue text.