fix(VBottomNav): shrink buttons when too narrow

fixes #4643

see #4547

Author: KaelWD

src/stylus/components/_bottom-navs.styl

@@ -39,6 +39,8 @@ theme(v-bottom-nav, "v-bottom-nav")
     text-transform: none
     opacity: .5
     width: 100%
+    // https://github.com/vuetifyjs/vuetify/issues/4643
+    flex-shrink: 1
 
     .v-btn__content
       flex-direction: column-reverse