LibJS: Add the Intl namespace object :^)

This is the start of implementing ECMA-402 in LibJS, better known as the
ECMAScript Internationalization API.

Much like Temporal this gets its own subdirectory (Runtime/Intl/) as
well as a new C++ namespace (JS::Intl) so we don't have to prefix all
the files and classes with "Intl".

https://tc39.es/ecma402/

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -25,6 +25,7 @@ namespace JS {
     P(EPSILON)                               \
     P(Function)                              \
     P(Infinity)                              \
+    P(Intl)                                  \
     P(JSON)                                  \
     P(LN10)                                  \
     P(LN2)                                   \