Support transientPrompt

[arthsmn]

What new feature should Elvish have?

Support cursor changes after a command is run, like this:

demo.mp4

Some shells that support it are fish, cmd (through clink), powershell and bash (through ble). This can be used directly or with an external prompt like starship.

Output of "elvish -version"

0.21.0

Code of Conduct

• I agree to follow Elvish's Code of Conduct.