Support Spacer in `<NMenu />`

[equt]

This function solves the problem (这个功能解决的问题)

To achieve effects like justify-content: space-between or arbitrary length of whitespace.

Expected API (期望的 API)

Almost the same as MenuDivider but without the default background.

https://github.com/tusen-ai/naive-ui/blob/9088b114c69d7311c617c95da115f7a7345095ba/src/menu/src/MenuDivider.tsx#L4-L15

[equt]

(please feel free to assign it to me if it's worth to have)

[07akioni]

Maybe we can expose some new theme overrides to support it?