Rename `Rule.rule` to `Rule.propertyName`

[oliverklee]

As suggested in a review: #1004 (comment)

https://developer.mozilla.org/en-US/docs/Web/API/CSS_Object_Model/CSS_Declaration

[JakeQZ]

It's already private, so we can do this without any deprecation issues.

[JakeQZ]

OTOH, there are the getRule and setRule methods, which would need corresponding getPropertyName and setPropertyName replacements, and a deprecation notice for the originals saying to use the replacements.