build(index): fix rollup error of mixing named and default exports

```
(!) Mixing named and default exports
Consumers of your bundle will have to use bundle['default'] to access the default export, which may not be what you want. Use `output.exports: 'named'` to disable this warning
```

This fixes the UMD bundle (in `dist`) as the library can only be
accessed via the default export.

Author: remarkablemark

index.js

@@ -19,10 +19,7 @@ function HTMLReactParser(html, options) {
   return domToReact(htmlToDOM(html, domParserOptions), options);
 }
 
-/**
- * Export HTML to React parser.
- */
-module.exports = HTMLReactParser;
+HTMLReactParser.domToReact = domToReact;
+HTMLReactParser.htmlToDOM = htmlToDOM;
 
-module.exports.domToReact = domToReact;
-module.exports.htmlToDOM = htmlToDOM;
+module.exports = HTMLReactParser;