feat: compress resized png icons

[ckerr]

When we generate resized icons – the $foo-32.png, $foo-64.png, $foo-128.png, $foo-256.png files – pipe them through pngquant.

I've intentionally used a lossy compressor here because:

• These are resized in a batch during npm run build, so CPU cost needs to be reasonable
• These icons are already derived, so a lossy compression doesn't alter the original files
• Choosing a lossy compressor gives a better speed/size result than lossless compression

Note that the original source icons are not part of this process. I've manually done lossless compression on them in #1077 and in this PR's first commit, but they are not piped through lossy compressors.

[zeke]

Nice! Any idea what the size differences are?