improvement 3: skip work if no new candidates are detected

- We already know a set of candidates from previous runs.
- We also already know a set of candidates that are invalid and don't
  produce anything.

This means that an incremental rebuild could give us a new set of
candidates that either already exist or are invalid.

If nothing changes, then we can re-use the compiled CSS.

This actually happens more often than you think, and the bigger your
project is the better this optimization will be.

For example:

```
// Imagine file A exists:
<div class="flex items-center justify-center"></div>
<button class="text-red-500">Delete</button>
```

```
// Now you add a second file B:
<div class="text-red-500 flex"></div>
```

You just created a brand new file with a bunch of HTML elements and
classes, yet all of the candidates in file B already exist in file A, so
nothing changes to the actual generated CSS.

Now imagine the other hundreds of files that already contain hundreds of
classes.

The beauty of this optimization is two-fold:

- On small projects, compiling is very fast even without this check.
  This means it is performant.
- On bigger projects, we will be able to re-use existing candidates.
  This means it stays performant.

Author: RobinMalfait

packages/tailwindcss/src/index.ts

@@ -187,13 +187,23 @@ export function compile(
 
   return {
     rebuild(newRawCandidates: string[]) {
+      let didChange = false
+
       // Add all new candidates unless we know that they are invalid.
+      let prevSize = allValidCandidates.size
       for (let candidate of newRawCandidates) {
         if (!designSystem.isInvalidCandidate(candidate)) {
           allValidCandidates.add(candidate)
+          didChange ||= allValidCandidates.size !== prevSize
         }
       }
 
+      // If no new candidates were added, we can return the original CSS. This
+      // currently assumes that we only add new candidates and never remove any.
+      if (!didChange) {
+        return compiledCss
+      }
+
       if (tailwindUtilitiesNode) {
         let previousAstNodeCount = designSystem.getAstNodeSize()
         let newNodes = compileCandidates(allValidCandidates, designSystem).astNodes