ensure we sort variants alphabetically by root

For incremental rebuilds we don't know all the used variants upfront,
which means that we can't sort them upfront either (what we used to do).

This code now allows us to sort the variants deterministically when
sorting the variants themselves instead of relying on the fact that they
used to be sorted before.

The sort itself could change slightly compared to the previous
implementation (especially when you used stacked variants in your
candidates), but it will still be deterministic.

packages/tailwindcss/src/variants.test.ts

@@ -1116,13 +1116,13 @@ test('min, max and unprefixed breakpoints', () => {
     }
 
     @media (width >= 1024px) {
-      .min-lg\\:flex {
+      .lg\\:flex {
         display: flex;
       }
     }
 
     @media (width >= 1024px) {
-      .lg\\:flex {
+      .min-lg\\:flex {
         display: flex;
       }
     }"

packages/tailwindcss/src/variants.ts

@@ -114,7 +114,7 @@ export class Variants {
     let compareFn = this.compareFns.get(aOrder)
     if (compareFn === undefined) return 0
 
-    return compareFn(a, z)
+    return compareFn(a, z) || a.root.localeCompare(z.root)
   }
 
   keys() {