#136: Speed up tailwindcss/classnames-order with officialSorting: true

Using chrome://inspect/, I found that the `createContext` function from
the Tailwind API took a lot of time to execute. It turned out that this
function was called more than only a few times, so I have optimized this
by adding a cache.

The cache uses a `WeakMap`, storing a `contextFallback` object for
every `config` object that is passed to `patchRegex`. I chose to use
`WeakMap` here because the Tailwind CSS config object may change over
time (see lib/util/customConfig.js), and this allows the garbage
collector to clean the cache when an old config is no longer used.

Interestingly enough, the time to execute the `create` function for an
ESLint rule is not included in the time reported by `TIMING=1`, as can
be seen in the details below. However, on my codebase (which I can't
share, unfortunately), this roughly halves the total runtime of the
`eslint .` command when using only `tailwindcss/*` rules.

Details of the run time:

Commands executed:
```
$ TIMING=1 node --inspect ./node_modules/.bin/eslint .
$ time node --inspect ./node_modules/.bin/eslint .
```

Before this commit:
```
Rule                                           | Time (ms) | Relative
:----------------------------------------------|----------:|--------:
tailwindcss/no-custom-classname                |   491.130 |    32.6%
tailwindcss/enforces-shorthand                 |   483.797 |    32.1%
tailwindcss/no-contradicting-classname         |   293.046 |    19.4%
tailwindcss/classnames-order                   |   206.097 |    13.7%
tailwindcss/migration-from-tailwind-2          |    16.470 |     1.1%
tailwindcss/enforces-negative-arbitrary-values |    16.250 |     1.1%
```

After this commit:
```
Rule                                           | Time (ms) | Relative
:----------------------------------------------|----------:|--------:
tailwindcss/no-custom-classname                |   482.266 |    34.0%
tailwindcss/enforces-shorthand                 |   434.553 |    30.6%
tailwindcss/no-contradicting-classname         |   276.010 |    19.5%
tailwindcss/classnames-order                   |   180.044 |    12.7%
tailwindcss/migration-from-tailwind-2          |    24.113 |     1.7%
tailwindcss/enforces-negative-arbitrary-values |    20.486 |     1.4%
```

Note that the timings above are single runs, not averages.
Repeated runs show that the _reported_ time for `classnames-order` does
not change significantly.

However, the output of `time` shows a different picture:
before this commit, the `wall` time is 19s and the `user` time is 30s;
after this commit, the `wall` time is 10s and the `user` time is 17s.
These timings are averaged over 5 runs.

ESLint config used (which, for testing, I have adapted in such a way
that _only_ the `tailwindcss/*` rules are executed):
```
{
  "env": {"browser": true, "es2020": true, "node": true},
  "extends": ["plugin:tailwindcss/recommended"],
  "overrides": [{"files": ["*.ts", "*.tsx"]}],
  "parser": "@typescript-eslint/parser",
  "parserOptions": {
    "ecmaFeatures": {"jsx": true},
    "sourceType": "module"
  },
  "plugins": ["react", "@typescript-eslint"],
  "settings": {
    "react": {"version": "detect"},
    "tailwindcss": {
      "cssFiles": ["src/**/*.css"],
      "officialSorting": true
    }
  }
}
```

Author: mpsijm

lib/rules/classnames-order.js

@@ -22,6 +22,8 @@ const createContextFallback = require('tailwindcss/lib/lib/setupContextUtils').c
 // messageId will still be usable in tests.
 const INVALID_CLASSNAMES_ORDER_MSG = 'Invalid Tailwind CSS classnames order';
 
+const contextFallbackCache = new WeakMap();
+
 module.exports = {
   meta: {
     docs: {
@@ -88,7 +90,14 @@ module.exports = {
     const removeDuplicates = getOption(context, 'removeDuplicates');
 
     const mergedConfig = customConfig.resolve(twConfig);
-    const contextFallback = officialSorting ? createContextFallback(mergedConfig) : null;
+    const contextFallback = officialSorting
+      ? (
+          // Set the created contextFallback in the cache if it does not exist yet.
+          contextFallbackCache.has(mergedConfig)
+            ? contextFallbackCache
+            : contextFallbackCache.set(mergedConfig, createContextFallback(mergedConfig))
+        ).get(mergedConfig)
+      : null;
 
     //----------------------------------------------------------------------
     // Helpers