i18n: Allow setting locale data by domain

[aduth]

Closes #5168
Cherry-picks 685eaf0 from #5481

This pull request seeks to enable a plugin to inject their own domain locale data into the global wp.i18n module, refactoring gutenberg_get_jed_locale_data as a general-purpose Jed locale formatter (removing previous assumptions of being the default domain).

Example:

wp.i18n.setLocaleData( { '': {}, foo: [ 'bar' ] }, 'myplugin' )
console.log( wp.i18n.__( 'foo', 'myplugin' ) );
// "bar"

Testing instructions:

Verify that there are no regressions in the display of default localized strings for Gutenberg.

Try the example above in your browser console while on the Gutenberg screen, verifying that "bar" is logged to the console.

[bordoni]

Would love to see that merged, we are trying to get that localization going for The Events Calendar compatibility. 🖤