Expand i18n text domain convention to cover all translation helpers #23

The new "i18n text domain" bullet in .github/copilot-instructions.md (added by #22) only names
__()/__esc() as requiring this plugin's text domain, but the file's existing i18n guidance
also treats __n(), __x(), __xn(), and __gettext() (and, in some plugins, __esc_n()) as
translation helpers. As written, the rule doesn't actually enforce a domain on every translated
string - a domainless plural/context call would still comply with the letter of the bullet.

Flagged by Copilot's review on #22; tracked here separately so the conventions PR isn't blocked
on it. Fix: either enumerate the full helper list in that bullet, or phrase it generically to cover
all translation/gettext helper calls.