This patch allows calendar and numberingSystem to be specified in
the options bag of the DateTimeFormat and NumberFormat constructors.
One use case for these options would be, when working with locales
which have two numbering systems and calendars in use--the UA default
may be the non-Western one, but in some contexts, it may be appropriate
to use the Western one. Currently, without the patch, it would be
necessary to parse the BCP 47 language tag in the application, but Intl
provides no library to do so. For example, this all occurs in this bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1370086

This patch validates the calendar and numbering system by comparing
them to the grammar allowed for Unicode extension tags, per the resolution
documented at tc39#175 (comment)

Related bug: tc39#105 . This patch leaves out "collation" because of a lack
of clear use cases.

- Convert calendar and numbering system to lowercase, analogously
  with canonicalizing language tags
- Reference the Unicode grammar, to avoid the need to keep up to
  date with notational changes from upstream

…-case-sensitivity-and-case-mapping

Adding case to map values properly when mapping to lower case