`input.command` and `saybutton.is-checked` need to be implemented

[ike709]

Ref parent issue https://github.com/OpenDreamProject/OpenDream/issues/167

As far as DMF functionality goes, these should be relatively high-priority as they're needed to make Paradise properly playable without needing to use the Say popup hotkey to chat.

TODO:

• [ ] Implement input.command.
• [x] Implement saybutton.is-checked. Done in https://github.com/OpenDreamProject/OpenDream/pull/1952