fix: add semicolon to avoid Uncaught TypeError on Webpack v5

See: #560

The compiled javascript pack is generated as follows:

```
...

/******/  // object to store loaded CSS chunks
/******/  var installedCssChunks = {
/******/    "common": 0
/******/  }/* webpack/runtime/jsonp chunk loading */
/******/  (() => {
/******/    // object to store loaded and loading chunks
/******/    // undefined = chunk not loaded, null = chunk preloaded/prefetched
/******/    // Promise = chunk loading, 0 = chunk loaded
/******/    var installedChunks = {
/******/      "common": 0
/******/    };

....
```

A missing semicolon after the closing `}` when defining
`installedCssChunks` causes the object `{ "common": 0}` to be
incorrectly interpreted as a function

Author: abhchand

src/index.js

@@ -251,7 +251,7 @@ class MiniCssExtractPlugin {
             Template.indent(
               chunk.ids.map((id) => `${JSON.stringify(id)}: 0`).join(',\n')
             ),
-            '}',
+            '};',
           ]);
         }