feat: fully onboard to astro:assets

[JoshuaKGoldberg]

PR Checklist

• [x] Addresses an existing open issue: fixes #222
• [x] That issue was marked as status: accepting prs
• [x] Steps in CONTRIBUTING.md were taken

Overview

• [x] Moves all the static images to ~/assets
• [x] Updates content collections to use assets / <Image />, and moves their images to ~/assets
• [x] Working builds with updated TS props and the like
• [x] Inline assets within blog posts
• [ ] Support for prefers-color-scheme-responsive images
• [x] Deleted old images (so most end up being just a move)
• [x] General polish & double-checking

The folks from #215 will be glad to see I removed the image fade-in stuff. I'll tack that on as a followup issue later.

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[JoshuaKGoldberg]

I want this in to get it off my todo list. Merging as-is 😄 and filing a followup issue for themed images.

Edit: #260