feat(b-table): add `selectAllRows()` and `clearSelected()` to thead/tfoot slot scopes (addresses #3901)#3907

jacobmllr95 merged 5 commits intodevbootstrap-vue/bootstrap-vue:devfrom

tmorehouse/table-head-scopebootstrap-vue/bootstrap-vue:tmorehouse/table-head-scopeCopy head branch name to clipboard