Add `/project-name issue list` slack command

This is a minor addition, but follows on from this (stale?) issue: #16512

The existing slack commands include /project-name issue search <query> which lists tickets from a linked project.

For our use case (managing BAU tasks in a shared slack channel) it would be really helpful to have a command to simply list all open issues in the linked project, without requiring a search query.

Command	Effect
/project-name issue list	Shows all open issues

I'm guessing this wouldn't be hard to implement since the "issue search" command already exists. Is this something I'd be welcome to submit an MR for?