Break mixins.less up into smaller modules.

Move all deprecated vendor prefix mixins to less/mixins/vendor-prefixes.less
Create mixins directory and move partials to that directory.

FIXES #12994

less/mixins/labels.less

@@ -0,0 +1,11 @@
+// Labels
+
+.label-variant(@color) {
+  background-color: @color;
+  &[href] {
+    &:hover,
+    &:focus {
+      background-color: darken(@color, 10%);
+    }
+  }
+}