Implement Temporal.PlainYearMonth.prototype.toLocaleString

[jedel1043]

Spec: https://tc39.es/proposal-temporal/#sec-temporal.plainyearmonth.tolocalestring

[alienx5499]

Hi @jedel1043,

I’ve opened a draft PR for this: #5098.

It follows the same approach as in #5080, reusing the shared format_date_time_locale pipeline and avoiding extra Intl.DateTimeFormat allocations.

The PR currently depends on the refactor in #5080. Once #5080 lands, I’ll rebase #5098 onto main so it only contains the Temporal changes for this issue.

If the direction looks good, please feel free to assign the issue to me.

Thanks!