refactor(flags): accept plural flag names through one normalize function

Every command accepts a flag's plural, such as --add-labels for
--add-label (D8), instead of per-command alias maps.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/issues.go

@@ -537,7 +537,6 @@ func (f *metadataFlags) register(cmd *cobra.Command) {
 	flags.StringSliceVar(&f.addAssignees, "add-assignee", nil, "Add assignees by `login`; @me for yourself")
 	flags.StringSliceVar(&f.removeAssignees, "remove-assignee", nil, "Remove assignees by `login`; @me for yourself")
 	flags.StringSliceVar(&f.setAssignees, "set-assignees", nil, "Replace all assignees; an empty value clears them")
-	flagAliases(cmd, map[string]string{"add-labels": "add-label", "remove-labels": "remove-label", "add-assignees": "add-assignee", "remove-assignees": "remove-assignee"})
 }
 
 // completeMetadataFlags completes the label and assignee flags of metadataFlags.