Add a `slash` subcommand to disable all commands

### What component of Red (cog, command, API) would you like to see improvements on?

Core

### Describe the enhancement you're suggesting.

A command such as `[p]slash clear` or `[p]slash disableall` should be created which disables all currently enabled slash commands.
Suggested by [seifcume](https://discord.com/channels/133049272517001216/133081046869737472/1307786856032636928).

### Anything else?

_No response_

The issue names no files, tests, or entry points. Begin by locating the core slash-command enable/disable handling and clarify whether the two proposed names are alternatives; done means a core command disables every currently enabled slash command.

Written by the indexing model from the issue text.