Currently empty, but we gotta start somewhere! This is the start of
implementing the Temporal proposal (currently stage 3).
I have decided to start a new subdirectory (Runtime/Temporal/) as well
as a new C++ namespace (JS::Temporal) for this so we don't have to
prefix all the files and classes with "Temporal" - there will be a lot.
https://tc39.es/proposal-temporal/
