extensionUtils: Add gettext convenience helpers

extensionUtils: Add gettext convenience helpers

We have initTranslations() for binding an extension's
gettext domain, but nothing to help with using gettext
from an extension.

Such help would be useful though, as an extension that
calls textdomain() like a normal application would
inadvertently changes the default domain for the whole
gnome-shell process.

Instead, extensions have to use domain-specific versions
of the gettext functions:

```js
const Gettext = imports.gettext.domain('my-extension');
const _ = Gettext.gettext;
```

Make this a bit easier by adding those functions directly
to the extensions object when initTranslations() is called,
then expose helper functions for calling them.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2594

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1941>