i18n: option to include current locale in locale_links filter?

[danburzo]

I think it would be beneficial to be able to include the current locale in the array of links returned by the locale_links filter. This would make sense for a variety of use cases:

1. for <link rel=alternate>, the recommendation is to include <link> tags for the current page, as well as for alternatives in other languages
2. it would provide direct access to the localized name of the current language, with the current locale at a stable position in the array, for various types of language switchers.

For #1, the workaround is provided as an example in the docs page — just print out the current page separately.

For #2 you can add iso-639-1 as a direct dependency and use iso639.getNativeName() on the current page's lang to get the localized name of the current language. Then override locale_links:

config.addFilter('locale_links_inclusive', function(url) {
	const locale_links = config.getFilter('locale_links');
	return locale_links(url).concat({
		lang: this.page.lang,
		url: this.page.url,
		label: iso639.getNativeName(this.page.lang.split('-').shift())
	}).sort((a, b) => a.label.localeCompare(b.label));
});

Would adding an option to the locale_links filter, e.g. locale_links(true) make sense to simplify this use-case?