feat: Add Locale.php class to set current locale 📡

[darcywong00]

Continues #384

This starts a Locale.php class which will expand to handle pointing to the localized.po files.
For now, it allows the current locale to be set with a query parameter like lang=fr-FR (previously was locale)

And confirmed with test code on keyboards/index.php

      <li>Current locale is <?= Locale::currentLocale() ?></li>      

[mcdurdin]

OK, but we do need to validate that it's an acceptable locale

[darcywong00]

TODO on follow-on PR once I add some .po files and workout how it fallsback to en strings.

[mcdurdin]

I haven't really considered how we set locale via url (keyman.com/en/foo vs keyman.com/foo?lang=en vs en.keyman.com/foo). (I like lang instead of locale but it's not critical). All of these options have pros and cons... bleagh :)

[darcywong00]

I haven't really considered how we set locale via url (keyman.com/en/foo vs keyman.com/foo?lang=en vs en.keyman.com/foo). (I like lang instead of locale but it's not critical). All of these options have pros and cons... bleagh :)

I changed the query parameter from locale to lang