This is not a calendar supported by ECMA-402, so let's not waste space
with its data.
Further, don't generate "gregorian" as a valid Unicode locale extension
keyword. It's an invalid type identifier, thus cannot be used in locales
such as "en-u-ca-gregorian".
This adds plumbing for the Intl.DateTimeFormat object, constructor, and
prototype.
Note that unlike other Intl objects, the Intl.DateTimeFormat object has
a LibUnicode structure as a base. This is to prevent wild amounts of
code duplication between LibUnicode, Intl.DateTimeFormat, and other
not-yet-defined Intl structures, because there's 12 fields shared
between them.