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/resize.less

@@ -0,0 +1,6 @@
+// Resize anything
+
+.resizable(@direction) {
+  resize: @direction; // Options: horizontal, vertical, both
+  overflow: auto; // Safari fix
+}