I18N: Introduce domain specific i18n gettext hooks.

Adding context to allow for more specific targeting. New hooks:

* `gettext-{$domain}`
* `gettext_with_context-{$domain}`
* `ngettext-{$domain}`
* `ngettext_with_context-{$domain}`

Fixes #49518.

Props geminilabs, garrett-eclipse, davidbaumwald, johnbillion, whyisjake.



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