Support creating a new subissue under the current issue

[drewwatkins11]

When creating a new issue, it would be helpful to have the option to add the new issue as a sub-issue under the currently open branch.

This can help track issues that are more complex than the current scope of work and allow them to be included in support workflows around branching or stacking, where additional branches in a stack are added to a sub-issue.

This would be added as a flag on the create command.

If this is an issue that is appreciated as part of the main codebase, I have a PR ready to submit.

[schpet]

yeah sounds good, i have used --parent for this before but not needing to copy and paste sounds appealing. curious how you implemented this (new command?) – interested in seeing the PR :-)