Fix polyfill in combination with unused CSS variable removal

[philipp-spiess]

This PR fixes an issue we noticed while investigating #17553, where the unused CSS variable removal didn't work properly when the theme variable it tried to remove was modified by a polyfill rule.

The way the bookkeeping for the unused CSS variable worked was that it tired to find the declaration inside it's parent after the traversal. However, the color-mix(…) polyfill has since then made changes to the declaration so it can't find it's position correctly anymore and will thus instead delete the last declaration of the node (this caused unrelated CSS variables to be eliminated while the ones with color-mix(…) were unexpectedly kept).

To fix this, we decided to apply the polyfills after any eventual deletions. This also ensures that no @supports query for the variables are created and simplifies the code a bit since all polyfills are now colocated.

Test plan

• Added a unit test for the example we discovered in [v4] Support --opacity-* theme configuration #17553
• Luckily the conditions of this seemed rare enough so that it doesn't cause any other of our tests to update.