Modernize showcase collection

[delucis]

This PR migrates our showcase collection to use more recent Astro features to simplify maintenance.

• Entries are now YAML data files instead of Markdown pretending to be data files
• Images are now resolved using the image() helper instead of a custom resolve function
• Images are now optimized with astro:assets — previously we committed two different resolutions to the repo and chose dynamically between them, now we just toggle the required width in <Image>
• Images are now directly colocated alongside the data files. Previously we have a src/content/showcase/_images/ directory for these. Now each site’s image file should appear alongside its data file in the file browser, e.g.
  
  There still lots and lots of files — not so much I can do about that. (I decided against a single large showcase.json data file thinking it would make finding a specific entry or seeing new entries in Git a little more clumsy.)
• I’ve updated our update:showcase package script to match this new setup
• In the process I also replaced the json-to-pretty-yaml dependency with the battle-tested yaml one. yaml is already used by other dependencies, so although json-to-pretty-yaml is laudibly minimal and lightweight, its more limited features weren’t worth working around.

Browser Test Checklist

I have tested this PR on at least three of the following browsers:

• Chrome / Chromium
• Firefox
• Android Firefox
• Safari
• iOS Safari

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[sarah11918]

I browsed both a few pages from the showcase, and a blog post using the showcase entries and all looks good!