Support for MyST markdown

[mgielda]

Hi! First of all, cool project! Looking at your extension syntax (https://kit-docs.svelteness.dev/docs/markdown/extensions#header-anchors) couldn't fail to notice this is very similar to the capabilities of MyST-markdown - https://myst-parser.readthedocs.io/en/latest/

MyST has a JS parser: https://executablebooks.github.io/mystjs/ and follows the CommonMark spec, with rst-compatible extensions.

The huge win is that MyST markdown also works seamlessly in a super-popular documentation framework, Sphinx, so it creates an interop later with a ton of preexisting documentation authored in rst or MyST - migrating to MyST from rst is a breeze.