Added support for DOCTYPE and storing ?xml

🆕 This releases allows parsing of full HTML documents.

Further testing with different kinds of XML data is required before being able to confidently mark this as release-worthy, and reaching the quality-mark of calling it a release. For now it should work in most development projects, but situations could occur where it fails to parse. Make sure to always check if the root node is null. As long as this is a pre-release, please stay in the Discord for notifications when the next release is made, as bug fixes are likely. Initial testing during livestream resolved several bugs.

Please read the README.md with markdown-compatible display or on the GitHub website for documentation.