Improvements to cancel command

[vanutp]

• [x] ability to stop running commands (implemented in 547c1c6d158e113f9ca63c8d0b87eb69a527d6af)
• [ ] ability to stop sync commands such as while True loop
• [x] make cancel and // permanent (do not reevaluate message when editing, if it was cancelled before) (implemented in bae83e2b0b9c9a788f99d921baf98c163b47e47e)