add mixin

.flexbox()
.flex(@flex)
.flex-direction(@direction)
.order(@order)
.justify-content(@justify-method)
.align-items(@align-method)
.flex-wrap(@wrap-method)
.align-self(@value)
.user-select()
.fixed-width(@value)
.fixed-height(@value)
.brimming()
.selection(@color:#fff,@bg:#39f)
.msgrid()
.msgrid-site(@column,@row,@colspan,@rowspan)