Fix ESM issue with webpack-css plugin (#340)

When we are using `size-limit` with` @size-limit/webpack-css` plugin `v10.0.1` we are having an issue and see this error running `size-limit` during plugin loading
```
ReferenceError: require is not defined in ES module scope, you can use import instead
This file is being treated as an ES module because it has a '.js' file extension and 'src/experiment
/size-limit-experiment/node_modules/@size-limit/webpack-css/package.json' contains \"type\": \"module\".
To treat it as a CommonJS script, rename it to use the '.cjs' file extension.\n
```

As a solution we use createRequire to require commonjs from esm module.

---

Also,
- adding webpack-css plugin as a workspace to global workspace package.json.
- checking that load-plugins() is not throwing an error when we load
  webpack-css
  (https://github.com/mendrew/size-limit/blob/f9e9a78c3f62be7061bfd25efa7e546f02de549c/packages/size-limit/test/load-plugins.test.js#L8-L16)

fixtures/zero-esbuild/package.json

@@ -4,6 +4,7 @@
   "devDependencies": {
     "@size-limit/esbuild": ">= 0.0.0",
     "@size-limit/file": ">= 0.0.0",
+    "@size-limit/webpack-css": ">= 10.0.1",
     "size-limit": ">= 0.0.0"
   },
   "size-limit": [

package.json

@@ -17,6 +17,7 @@
     "@size-limit/preset-small-lib": "workspace:^",
     "@size-limit/time": "workspace:^",
     "@size-limit/webpack": "workspace:^",
+    "@size-limit/webpack-css": "workspace:^",
     "@size-limit/webpack-why": "workspace:^",
     "@vitest/coverage-v8": "^0.34.6",
     "clean-publish": "^4.2.0",

packages/size-limit/test/load-plugins.test.js

@@ -9,9 +9,10 @@ describe(`load-plugins`, () => {
     let cwd = join(__dirname, '..', '..', '..', 'fixtures', 'zero-esbuild')
     let result = await loadPlugins(await readPkgUp(cwd))
     expect(result.isEmpty).toBe(false)
-    expect(result.list.length).toBe(2)
+    expect(result.list.length).toBe(3)
     expect(result.has('esbuild')).toBe(true)
     expect(result.has('file')).toBe(true)
+    expect(result.has('webpack-css')).toBe(true)
     expect(result.has('plugin')).toBe(false)
   })
 

packages/webpack-css/index.js

@@ -1,4 +1,7 @@
 import CssMinimizerPlugin from 'css-minimizer-webpack-plugin'
+import { createRequire } from 'node:module';
+
+const require = createRequire(import.meta.url);
 
 const cssRule = {
   exclude: /\.module\.css$/,