Support for MSC4391: Simplified in-room bot commands

[nexy7574]

Describe the problem

MSC4391: Simplified in-room bot commands provides a way to bots to create custom commands, generally expected to be represented as slash commands or similar in clients. Since Sable already has a slash commands system, this would probably integrate quite nicely. It also means the UX around using bot commands massively improves, since MSC4391 structures the data sent to bots, meaning string parsing woes are alleviated.

Describe the solution you'd like

Support for MSC4391 to allow bots to register commands that users can use.

Alternatives considered

No response

Additional context

Issue context in Matrix room: https://matrix.to/#/!a6sXbRuOyyc7MKutmy:sable.moe/%24CWJXBRiiQbtYVUE3p1S5nb1ogGUJ3Adkp5la1LBiqX4?via=nexy7574.co.uk&via=sable.moe&via=matrix.org

[dozro]

#156 should probably be done first to have a proper interface for what we then get and need to render