Preserve custom properties in keyframes

[adamwathan]

Closes #16374

Ensure we don't remove custom properties from within @keyframe declarations.

[philipp-spiess]

@thecrypticace @RobinMalfait I was wondering if we can clean this up a bit by starting to use an AST walk instead and relying on the parentNode being set correctly to see wether we're inside a keyframe to avoid touching any properties. WDYT?

The code to remove nodes inside a context.reference boundary would become more annoying since we'll have to remove every sibling node instead of removing the whole context node (as context nodes are invisible for traversal).

[RobinMalfait]

That's an option, but by the time we see a declaration, the parent node will never be the @keyframes node, but will be the intermediate rule:

@kefyrames {
  0% { /* <- this will be the parent */
    --foo: red;
  }
}

Or do you mean that at the the time we reach @keyframes, we do a walk in this block, and then mark all found declarations as "don't touch me"?

[philipp-spiess]

Yeah I was thinking of doing a return WalkAction.Skip maybe.