Investigate current handling of !important CSS declarations

[kasperisager]

When computing styles, Alfa traverses the rule tree (packages/alfa-cascade/src/rule-tree.ts) for every element in order to determine the rules that match an element, in decreasing order of priority. However, style computation currently fails to account for individual property declarations that specify the !important flag. Servo deals with this directly in the rule tree by seemingly splitting out !important declarations into separate nodes in the tree in order to correctly position them: https://doc.servo.org/style/rule_tree/struct.RuleTree.html#method.insert_ordered_rules_with_important. We might need to do something similar.