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/text-overflow.less

@@ -0,0 +1,8 @@
+// Text overflow
+// Requires inline-block or block for proper styling
+
+.text-overflow() {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}