Update Remix builds to Vite + single-fetch (unstable_defineLoader)

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: prepares us to fix 🛠 Tooling: use Remix prerender feature  #96
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Updates to Remix 2.9.2, switches to use Vite instead of the older esbuild compiler, and enables the single fetch feature. This will unlock the ability to test out the new unreleased pre-alpha experimental prerender feature if we want 😉

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Recreation of @brophdawg11's #97.