i18n: Add Irish (ga_IE) translation (#7650)

This PR introduces support for Irish (Gaeilge), the first official
language of Ireland and an EU working language.

The translation file was initialized using the standard gettext tooling:

` msginit -i po/com.mitchellh.ghostty.pot -l $LANG -o "po/$LANG.po"`

The locale code `ga_IE` follows ISO standards for Gaeilge as spoken in
Ireland.

I'm happy to volunteer as the ongoing maintainer of the Irish
translation and will keep it up to date as Ghostty evolves.

Go raibh maith agat!

src/os/i18n.zig

@@ -46,6 +46,7 @@ pub const locales = [_][:0]const u8{
     "es_AR.UTF-8",
     "pt_BR.UTF-8",
     "ca_ES.UTF-8",
+    "ga_IE.UTF-8",
 };
 
 /// Set for faster membership lookup of locales.