Cascader add `show-all-levels` option

[1sm23]

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

What problem does this feature solve?

Display full labels or just last selected option's label

What does the proposed API look like?

props:{
  showAllLevels: {
    type: Boolean,
    default: true
  }
}

Element-plus-cascader

[1sm23]

displayRender solved my problem.

[github-actions[bot]]

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