Describe the feature or problem you’d like to solve

I want completions for --state flag of gh pr list/gh issue list command.

Expected behavior is:

$ gh pr list --state [TAB]
all  closed  merged  open

It prevent wrong input such as gh pr list --state opened, gh issue list --state close, etc...
(I wasted many time by these typos)

Proposed solution

We can use cobra's completion support (RegisterFlagCompletionFunc) in the same way as #3628.

I confirmed in my branch which works good on zsh.

Diff is here: 936d6e1...autopp:complete-state-flag
If this approach is okey, I'll submit it as a PR

Additional context

n/a