This is especially helpful where we already pass StringView literals
and only compare them with others, e.g. overflow and largest/smallest
unit, in which case there's no need to actually allocate a string.
When overriding visit_edges() in a JS::Object subclass, we must make
sure to call the base class visit_edges(), or the object's Shape (and
any properties) will not get marked.
This commit adds the PlainYearMonth object itself, its constructor and
prototype (currently empty), and the CreateTemporalYearMonth and
ISOYearMonthWithinLimits abstract operations.
