Add :done selector

Adds selector `:done` that groups items that are done according to
`org-done-keywords`.

README.org

@@ -194,6 +194,7 @@ These selectors take one argument alone, or multiple arguments in a list.
 +  =:date= :: Group items that have a date associated. Argument can be =t= to match items with any date, =nil= to match items without a date, or =today= to match items with today’s date. The =ts-date= text-property is matched against.
 +  =:deadline= :: Group items that have a deadline. Argument can be ~t~ (to match items with any deadline), ~nil~ (to match items that have no deadline), ~past~ (to match items with a deadline in the past), ~today~ (to match items whose deadline is today), or ~future~ (to match items with a deadline in the future). Argument may also be given like ~before DATE~ or ~after DATE~ where DATE is a date string that ~org-time-string-to-absolute~ can process.
 +  =:effort<= :: Group items that are less than (or equal to) the given effort. Argument is a time-duration string, like ~5~ or ~0:05~ for 5 minutes.
++  =:done= :: Group items that are done according to ~org-done-keywords~.
 +  =:effort>= :: Group items that are higher than (or equal to) the given effort. Argument is a time-duration string, like ~5~ or ~0:05~ for 5 minutes.
 +  ~:file-path~ :: Group items whose buffers' filename paths match any of the given regular expressions.
 +  =:habit= :: Group habit items (items which have a =STYLE: habit= Org property).

org-super-agenda.el

@@ -778,6 +778,13 @@ keyword, or `nil' to match only non-todo items."
           (_ ;; Oops
            (user-error "Argument to `:todo' must be a string, list of strings, t, or nil"))))
 
+(org-super-agenda--defgroup done
+  "Group items that are done according to `org-done-keywords'."
+  :section-name "Done items"
+  :test (org-super-agenda--when-with-marker-buffer (org-super-agenda--get-marker item)
+          (when-let ((state (org-get-todo-state)))
+            (member state org-done-keywords))))
+
 (org-super-agenda--defgroup ancestor-with-todo
   "their earliest ancestor having the to-do keyword"
   ;; TODO: Add tests.

org-super-agenda.info

@@ -422,6 +422,8 @@ list.
      Group items that are less than (or equal to) the given effort.
      Argument is a time-duration string, like ‘5’ or ‘0:05’ for 5
      minutes.
+‘:done’
+     Group items that are done according to ‘org-done-keywords’.
 ‘:effort>’
      Group items that are higher than (or equal to) the given effort.
      Argument is a time-duration string, like ‘5’ or ‘0:05’ for 5