This last part to support "calendar" requires a bit more code.
`GetCalendarDisplayName()` performs the following steps:
1. Validate the input matches the `type` nonterminal, per
CanonicalCodeForDisplayNames, step 4.a.
2. Then convert it to lower case, because ICU expects lower case names and it
needs to be in lower case for `LanguageTag::replaceUnicodeExtensionType()`.
3. Replace calendar name aliases with the preferred name, e.g. use "ethioaa"
instead of "ethiopic-amete-alem".
4. Check the input isn't too large, just in case ICU somehow doesn't properly
handle large string inputs.
5. Retrieve the old-style value of the calendar name, for example "gregorian"
for the calendar name "gregory".
6. And finally call `uldn_keyValueDisplayName` to search for the localised name.
Our ICU data file doesn't yet include calendar names. To include calendar names
we need to enable them in "data_filter.json", rebuild the ICU data file, and
then reapply the update for tzdata 2021a. (Reapplying the tzdata changed
"intl/tzdata/GIT-INFO", but otherwise didn't change any time zone files.)
Adding the calendar names to the ICU data file results in a slight increase of
the overall data file size (79,184 bytes).
The existing "calendar.js" test file was renamed to "month-calendar.js", so
the test file for the new "calendar" type can be named "calendar.js".
Differential Revision: https://phabricator.services.mozilla.com/D116008
André Bargull