Previously @media rule conditions could be updated by assigning to
`conditionText`. This change aligns our implementation with the CSSOM
specification, which says `CSSConditionRule.conditionText` should be
read-only.
NewAKString is effectively the default for any new IDL interface, so
let's mark this as the default behavior. It also makes it much easier to
figure out whatever interfaces are still left to port over to new AK
String.