Add application command localization support.

[expeehaa]

Summary

This PR adds support for registering and editing name and description localizations of application commands and their options. I took a slightly different approach than in #190, because I don’t like the additional builder.

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Added

• Support for registering and editing the name_localizations and description_localizations properties of application commands and their options
• Example usage of application command localizations in the slash commands example
• Support for setting with_localizations when calling Discordrb::Bot#get_application_commands.

[expeehaa]

I’ll try to do that and rebase the branch soon.