handling of 'islamic' and 'islamic-rgsa' calendars

[fabon-f]

As far as I understand, there is a consensus that Temporal doesn't accept nor do fallback for 'islamic' and 'islamic-rgsa' calendars, as specified in tc39/proposal-intl-era-monthcode#46 . But currently temporal_rs accepts these calendars, and I believe this is spec-incompliant.

https://github.com/boa-dev/temporal/blob/d125b624245a39556e1de9aff9fce174365aadde/src/builtins/core/calendar.rs#L144-L159 https://github.com/boa-dev/temporal/blob/d125b624245a39556e1de9aff9fce174365aadde/src/builtins/core/calendar.rs#L106-L108