[Feature Request] VDataTable `mustSort` can be an array of keys

[VividLemon]

Problem to solve

When building out a multi-sort system of my own, inspired by Vuetify, I found myself wondering "what if someone wants to 'mustSort' a specific field, if in a multi-sortable table?". Under what circumstance one would want to do this, idk. But perhaps it could be a useful addition to Vuetify

Proposed solution

Feel free to copy my solution: https://github.com/VividLemon/bootstrap-vue-next/blob/e9436efd7cd635787bdff17a42cab33268fef1d0/packages/bootstrap-vue-next/src/components/BTable/BTable.vue#L540