I18N: localize 'jS' to day+ordinal in 'date_i18n'

[jayaddison]

The built-in PHP NumberFormatter can localize numbers to include ordinal suffixes; however the return value always includes both the numeric value and also the suffix -- e.g. it is not currently possible to request only the suffix.

In the case of date formatting, it's fairly typical that if ordinals are expected, then they will be output for the calendar day number (e.g. the 11 in 2024-01-11) - and that the suffix should appear directly adjacent (e.g. 11th).

Day-of-month should be restricted to values no larger than 31, and should all be integer values, so we shouldn't need to worry about any locale-specific introduction of thousands or decimal place markers.

Based on the above, we can implement limited/restricted localization of day-number-plus-ordinal-suffix in the WordPress date_i18n function by special-case detecting an S (ordinal format) character symbol immediately preceded by a j (day of the month) format code.

Trac ticket: https://core.trac.wordpress.org/ticket/22225

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[jayaddison]

(also: the documentation will need to be updated alongside this modification)

[jayaddison]

The error that appears is:

With the latest pushed commits in this PR, this error is resolved - a NumberFormatter is only constructed if the class exists (in the global namespace) according to check_exists.

One other question I have is whether the logic, if acceptable (it's not exactly an elegant solution) should use get_locale or get_user_locale -- I'm not sure about that.