build(docs): use custom Vite plugin to develop docs

[metonym]

I want to avoid syncing the README with the deployed demo site (e.g., #54).

Since the demo site is almost a verbatim reflection the README, it makes sense to build the docs site from the README.

This use a custom Vite plugin that preprocesses the README before it's processed by the Svelte Vite plugin.

It uses marked along with a couple of plugins to do the heavy lifting. In addition, it designated a watchDir, in which component cases can be added or updated.

These cases are then referenced as inline comments in the README. As Markdown, they are invisible, but in the docs site, they render the live example.

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Some additional work following this:

• Performance: pre-render the HTML and re-hydrate interactivity
• Performance: de-couple component instances from the bulk of the markup (avoid including code strings in JS bundle)
• Extract the custom plugin into an NPM package
• Write unit tests using the example component cases