Localization: internal change - use the "meta" package to specify the list of supported languages for all 3 currently-localized packages

This proposed change would mean that future l10n contributions wouldn't have to modify the `packages/ace-XXX/src/l10n/localize.js` source code for each of the 3 currently-localised packages. For example, see this recent Pull Request:

https://github.com/daisy/ace/pull/246/files#diff-38adeefea9ae9024cab3ce220f321259

Caveat: conditional/dynamic `require()` calls (which would be needed in order to iterate the array of supported locales) may be problematic with some code bundlers, at build time.

To be investigated.