i18n: Add domain argument to localization functions

[aduth]

Gutenberg was localized without explicit domain in its JavaScript gettext implementation with the following assumptions in mind:

• It would make its way to core, where the default strings use the inferred empty domain
• At worst, a domain could be "injected" as part of the string extraction step as a parameter to the extraction utility

Since these assumptions do not hold true for plugin code, and we should seek to open up localization tools more broadly, we should introduce a domain parameter to the localization functions.

As part of this, we may consider being explicit in specifying gutenberg domain for strings within Gutenberg, since we do this in PHP code and Gutenberg is still technically a plugin. That said, the plugin manifest does not specify the text domain.

This should not be a very difficult task, as it's merely a matter of adding the argument and using the domain-prefixed Jed functions. For example:

Before:

export function __( text ) {
	return getI18n().gettext( text );
}

(Reference)

After:

export function __( text, domain ) {
	return getI18n().dgettext( domain, text );
}

(Repeat for _x, _n, _nx)

[Rahmon]

dgettext function has 'domain' parameter. So the correct should be "return getI18n().dgettext( domain, text );". Right?

[aduth]

Yes, silly oversight on my part. Thanks for the correction! I've updated the original text.

[ocean90]

That said, the plugin manifest does not specify the text domain.

You mean Text Domain and Domain Path? Both can be omitted if the plugin is in the plugin directory and requires at least WordPress 4.6. See https://developer.wordpress.org/plugins/internationalization/how-to-internationalize-your-plugin/#domain-path and https://developer.wordpress.org/plugins/internationalization/how-to-internationalize-your-plugin/#loading-text-domain.