Cold start performance improvements (#20427)

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## Summary

Hi guys, thanks for you great work!

I work on performance improvements on
https://github.com/schoero/eslint-plugin-better-tailwindcss project.
Part of issues can be fixed on the tailwind side only.

This is a first fix, I have a bigger one in mind, it would require a
small additional public API method - out of scope of this PR.

Please let me know what you think.

## Finding

`getVariantOrder()` re-sorts all parsed variants with the (expensive)
variant comparator on every call, and it is called by every
compileCandidates() invocation — per build pass, per @apply
substitution, and hundreds of times during candidate canonicalization
via the variant signature caches. Since parsedVariants is append-only,
the computed order only changes when a new variant is parsed, so we can
cache the result and invalidate on parsedVariants.size. This makes cold
canonicalization (IntelliSense, lint plugins) faster and removes
repeated sorting from @apply substitution and incremental rebuilds.

## Test plan

Tested at https://github.com/smnbbrv/better-tailwindcss-bench . The
relevant part is the `epbt-now / tw-patched`

**plugin cost** (ms, cold run net of parse baseline)

| codebase | epbt-now / tw-now | epbt-patched / tw-now | epbt-now /
tw-patched | epbt-patched / tw-patched |
| ---------- | ----------------: | --------------------: |
--------------------: | ------------------------: |
| mixed | 6292 | 3965 (-37%) | 5395 (-14%) | 3595 (-43%) |
| repetitive | 5279 | 2425 (-54%) | 4819 (-9%) | 2124 (-60%) |
| unique | 6096 | 5458 (-10%) | 5507 (-10%) | 5295 (-13%) |

packages/tailwindcss/src/design-system.ts

@@ -75,6 +75,8 @@ export function buildDesignSystem(
   let variants = createVariants(theme)
 
   let parsedVariants = new DefaultMap((variant) => parseVariant(variant, designSystem))
+  let cachedVariantOrder: Map<Variant, number> | null = null
+  let cachedVariantOrderSize = -1
   let parsedCandidates = new DefaultMap((candidate) =>
     Array.from(parseCandidate(candidate, designSystem)),
   )
@@ -188,6 +190,11 @@ export function buildDesignSystem(
     },
 
     getVariantOrder() {
+      // parsedVariants only grows, so the cache should be reset only when the size changes
+      if (cachedVariantOrder !== null && cachedVariantOrderSize === parsedVariants.size) {
+        return cachedVariantOrder
+      }
+
       let variants = Array.from(parsedVariants.values())
       variants.sort((a, z) => this.variants.compare(a, z))
 
@@ -209,6 +216,9 @@ export function buildDesignSystem(
         prevVariant = variant
       }
 
+      cachedVariantOrder = order
+      cachedVariantOrderSize = parsedVariants.size
+
       return order
     },