Add `ContextMenuCommand` message type (#1472)
Add `ContextMenuCommand` message type (#1472)

This commit adds support to context menu command message type, as per https://github.com/discord/discord-api-docs/pull/3651.

Note that the message type 20, which was previously named `ApplicationCommand`, will be renamed to `ChatInputCommand` in the `next` branch, as it is a breaking change.
acdenisSK/serenity,Flat/serenity