Adopt svelte-kit package and add Markdoc.svelte

[bayre]

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This is a first attempt at adding Markdoc.svelte into the mix. I have rejigged the repo as per svelte-kit package, but if you can think of a cleaner way to bundle the component and the preprocessor then I'd be happy to abandon this approach.

These things remain on the todo list:

• [ ] Updated README
• [ ] Tests?
• [ ] Types?

[bayre]

Hey @joshnuss — I've tackled most of your comments bar the idea of using Markdoc.svelte in the preprocessor. I need to wrap my noggin around the API for that. Still needs tests too 🙃