This doesn't quite do what I suggested. It looks up the case-sensitivity of the property every time the setter is called, which is not necessary. Instead the case-sensitivity should be looked up at prepare-time, so that when we generate lib/generated/propertyDescriptors.js, the calls look like createGenericPropertyDescriptor("animation-name", { caseSensitive: true }).

Looking back at the PR, I wondered if it would be better to combine generatePropertyDefinitions.mjs and generatePropertyDescriptors.mjs into a single file.
WDYT?

I think this is a good idea. It is very related to the discussion at #304 (comment).

But, let me merge this first as it is a good improvement by itself.