Wait for the session to be ready when executing a command without opening the tab

[cmarmo]

Fixes #278.

This pull request waits for the session to be ready when creating a Kernel Output panel to execute a command.

Thank you for considering it.

[jtpio]

Thanks @cmarmo!

#280 should already help a bit with the broken CI, but there may be still be some failing checks. Will try to have a look soon.

[cmarmo]

@jtpio I think the failing check was my responsibility: I didn't realize that the check verifies that the lines referred to into the README.md should correspond to the actual code.
I hope this is fixed now.

[jtpio]

Thanks @cmarmo.

This last commit on main also seems to have failing checks: 4585387

Based on #280 (review), it looks like this extension examples repo needs to be updated to use the latest extension template to fix the remaining checks.