This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.
One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.
It's not at all obvious how we need three different array-like types.
This change to the Patterns documentation attempts to explain why they
exist, how they differ (mostly in allocation behavior) and what their
use cases are. This builds on #11844 which fixates and tests the
hereby-described allocation behavior of FixedArray.
