Create a minimal test case for webpack-contrib/mini-css-extract-plugin#489

Author: klesun

README.md

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+Steps to reproduce `Callback was already called` in
+[mini-css-extract-plugin](https://github.com/webpack-contrib/mini-css-extract-plugin)when you try to use it with 
+[html-webpack-plugin](https://github.com/jantimon/html-webpack-plugin) to minimize css referenced in `<link href='...'/>`  
+
+- Clone this repo
+- Run `npm ci`
+- Run `node node_modules/webpack/bin/webpack.js`
+
+Will fail with this error:
+```$
+Error: Callback was already called.
+    at throwError (/mnt/winda/gits/mcep_issue_489/node_modules/neo-async/async.js:16:11)
+    at /mnt/winda/gits/mcep_issue_489/node_modules/neo-async/async.js:2818:7
+    at process._tickCallback (internal/process/next_tick.js:61:11)
+```

index.css

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+.item {
+    color: red;
+}

index.html

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+<link href="./index.css"/>
+<div class="item">some text</div>

index.js

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+alert('JS loaded!');