LibJS: Add the Temporal namespace object :^)

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/

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -46,6 +46,7 @@ namespace JS {
     P(SQRT2)                                 \
     P(String)                                \
     P(Symbol)                                \
+    P(Temporal)                              \
     P(UTC)                                   \
     P(abs)                                   \
     P(acos)                                  \