Emit --tw-drop-shadow property

[philipp-spiess]

Fixes #16461

Ensure we also emit @property --tw-drop-shadow.

Test plan

<div class="drop-shadow-2xl size-72 bg-white">
  <div class="size-48 saturate-100 bg-white"></div>
</div>

now only drops one shadow (screenshot from Vite playground):

[thecrypticace]

We should add a UI test for this if we can