Convert site to SvelteKit

[benmccann]

This is https://github.com/pngwn/MDsveX/pull/466 with a few changes:

• the REPL fixes are no longer included since those were merged separately
• it upgrades a couple of Rollup plugins. The build was bombing out on the latest master until I did this
• upgrades to SvelteKit 2

[changeset-bot[bot]]

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Latest commit: cd615ed23225bf794d817a17045f59a8668379b5

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[pngwn]

Thanks Ben!