Require slug in Partial<Block> type

[jacobsfletch]

By using a Partial type for field blocks, all properties become optional including those which are truly required for Payload to operate, namely slug. We need to explicitly require this field and any others that might be necessary.

From this Discord thread: https://discord.com/channels/967097582721572934/1088878072180244510

[jacobsfletch]

This plugin is now being maintained in the Packages Directory of the Payload Monorepo. This repo will soon be archived and all open issues including this one will be closed. This issue has already been added to this open discussion, though, so that it will not get lost. Please refer to that discussion for more details and to continue the conversation.