#136: Speed up tailwindcss/enforces-shorthand

Using chrome://inspect/, I found that the `patchRegex` function took a
lot of time to calculate. It looked like it was often called with the
same parameters, which I have optimized by adding a cache.

The cache uses a `WeakMap`, storing a plain string-to-string 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.

On my codebase (which I can't share, unfortunately), this greatly
reduces the run time of the `tailwindcss/enforces-shorthand` rule.

Details of the run time:

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

Before this commit:
```
Rule                                           | Time (ms) | Relative
:----------------------------------------------|----------:|--------:
tailwindcss/enforces-shorthand                 |  2444.777 |    70.0%
tailwindcss/no-custom-classname                |   487.850 |    14.0%
tailwindcss/no-contradicting-classname         |   289.491 |     8.3%
tailwindcss/classnames-order                   |   229.961 |     6.6%
tailwindcss/migration-from-tailwind-2          |    20.250 |     0.6%
tailwindcss/enforces-negative-arbitrary-values |    17.669 |     0.5%
```

After this commit:
```
Rule                                           | Time (ms) | Relative
:----------------------------------------------|----------:|--------:
tailwindcss/enforces-shorthand                 |   564.097 |    32.8%
tailwindcss/no-custom-classname                |   531.374 |    30.9%
tailwindcss/no-contradicting-classname         |   341.007 |    19.8%
tailwindcss/classnames-order                   |   238.749 |    13.9%
tailwindcss/enforces-negative-arbitrary-values |    22.539 |     1.3%
tailwindcss/migration-from-tailwind-2          |    20.349 |     1.2%
```

Note that the timings above are single runs, not averages, but repeated
runs show a similar pattern.

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
    }
  }
}
```

lib/util/groupMethods.js

@@ -261,14 +261,23 @@ function generateOptions(propName, keys, config, isNegative = false) {
   }
 }
 
+const cachedRegexes = new WeakMap();
+
 /**
  * Customize the regex based on config
  *
  * @param {String} re Regular expression
  * @param {Object} config The merged Tailwind CSS config
- * @returns {String} Patched version with config values and additinal parameters
+ * @returns {String} Patched version with config values and additional parameters
  */
 function patchRegex(re, config) {
+  if (!cachedRegexes.has(config)) {
+    cachedRegexes.set(config, {});
+  }
+  const cache = cachedRegexes.get(config);
+  if (re in cache) {
+    return cache[re];
+  }
   let patched = '\\!?';
   // Prefix
   if (config.prefix.length) {
@@ -281,7 +290,7 @@ function patchRegex(re, config) {
   const resArray = [...res];
   const props = resArray.map((arr) => arr[1]);
   if (props.length === 0) {
-    return `${patched}(${replaced})`;
+    return cache[re] = `${patched}(${replaced})`;
   }
   // e.g. backgroundColor, letterSpacing, -margin...
   props.forEach((prop) => {
@@ -336,7 +345,7 @@ function patchRegex(re, config) {
     const opts = generateOptions(absoluteProp, keys, config, isNegative);
     replaced = replaced.replace(token, opts);
   });
-  return `${patched}(${replaced})`;
+  return cache[re] = `${patched}(${replaced})`;
 }
 
 /**