Vue3 selected-option-container-wrapper

[chuntin]

Is your feature request related to a problem? Please describe.
People needs to show number of elements selected in order to simplify a big amount of elements selected to keep the design fit.

Describe the solution you'd like
A slot wrapper of selected-option-container can be the better solution in order to not altered the actual code.

Describe alternatives you've considered
Can be a prop with the options number to be showed and text after this options -> We need more logic.

Additional context
There are a possible solution in a PR for master #1361
It's very easy to implement and everyone can use like they want.
Can be this PR approved for beta branch? or do you want I write this PR for that branch?

[Kronhyx]

@sagalbot please we need this