Add command to clear current output in REPL

[failable]

Summary

Although one can click the cross button to clear current output, adding command will also be convenient. repl::ClearOutputs will clear all the output.

[rgbkrk]

Just to clarify, are you asking for an action to clear the output of the current cell?

[failable]

Yes！

[github-actions]

Hi there! 👋
We're working to clean up our issue tracker by closing older bugs that might not be relevant anymore. If you are able to reproduce this issue in the latest version of Zed, please let us know by commenting on this issue, and it will be kept open. If you can't reproduce it, feel free to close the issue yourself. Otherwise, it will close automatically in 14 days.
Thanks for your help!