PR listing: add --fields & expose additional fields #415

This PR adds the --fields flag to tea pr ls (#342), and exposes more fields specific to the PullRequest type:

   --fields value, -f value   Comma-separated list of fields to print.
                              Available values:
                              index,state,author,author-id,url,title,body,mergeable,base,base-commit,head,diff,patch,created,updated,deadline,assignees,milestone,labels,comments
                              (default: "index,title,state,author,milestone,updated,labels")

Thanks!
#400 also proposes to implements this, though in a different way.
As described in that PR, there's problems with that approach, and currently I tend to remove the tea pr ls changes from #400, instead aiming to add more filters to the API in Gitea & SDK in the long run, in the end favoring this PR.

As per Discord chat with @noerw: I added support for additional fields to make it more similar to the issues command. Are there any other fields that would be nice to have for PRs?

Also, I'm not totally happy with these changes. The formatting related code for issues and pulls are ~100% duplicates.

We could partially integrate #258 in this PR. conflicts? is already represented by the additional field mergeable. base/head branch are available and only needs to be added to the optional fields.
Regarding reviews (count) we'd need to do an extra rest request to retrieve all reviews of that PR to count them.

 		for _, label := range x.Labels {
 			if _, ok := labelMap[label.ID]; !ok {
 				labelMap[label.ID] = formatLabel(label, !isMachineReadable(output), "")
 				labelMap[label.ID] = label

PR listing: add --fields & expose additional fields #415

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