site: allow multiple blog authors

[geoffrich]

Opened against the svelte-4 branch since I think that's the only one where site deploys work, but lmk if it should be main instead.

This PR allows specifying multiple authors on a blog post by adding comma separation in the frontmatter, e.g.

---
title: Streaming, snapshots, and other new features since SvelteKit 1.0
description: Exciting improvements in the latest version of SvelteKit
author: Geoff Rich, Rich Harris
authorURL: https://geoffrich.net, https://twitter.com/Rich_Harris
---

Updated the streaming blog post from earlier this year to give credit to Rich, since he wrote the "how does this work" section.

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[benmccann]

I wonder if we should use a different delimiter like ; or | so that we don't break in case someone writes their name like Doe, John or John Doe, MD, PhD

[Rich-Harris]

I'm inclined to file that under 'cross that bridge when we come to it'