feat: add support for full name rendering in completion

Add new display transform function for full names.

Generalize existing `citar--shorten-names` to `citar--render-names`
to allow code-sharing between completion rendering of family names
and full names.

Refs: emacs-citar#805

citar.el

@@ -191,6 +191,7 @@ references as a string."
 (defcustom citar-display-transform-functions
   ;; TODO change this name, as it might be confusing?
   `((sn . (citar--shorten-names))
+    (fn . (citar--full-names))
     (etal . (citar--shorten-names 3 "&")))
   "Configure transformation of field display values from raw values.
 
@@ -1416,20 +1417,14 @@ See the documentation for `citar-add-file-sources' for more details."
 
 ;;; Format and display field values
 
-(defun citar--shorten-name-position (namelist name)
+(defun citar--render-name-position (namelist name)
   "Return NAME position in a NAMELIST."
   (+ (seq-position namelist name) 1))
 
-(defun citar--shorten-name (name)
-  "Return family NAME in `family, given' string.
+(defun citar--render-names (namestr name-func &optional truncate andstr)
+  "Return a list of names from a list of full NAMESTR.
 
-Otherwise, return as is."
-  (car (split-string name ", ")))
-
-(defun citar--shorten-names (namestr &optional truncate andstr)
-  "Return a list of family names from a list of full NAMESTR.
-To better accommodate corporate names, this will only shorten
-personal names of the form \"family, given\".
+Each name will be processed by NAME-FUNC.
 
 With an integer TRUNCATE, will shorten the list, and ANDSTR will
 replace last comma."
@@ -1439,8 +1434,8 @@ replace last comma."
          (tnamelength (length tnamelist)))
     (mapconcat
      (lambda (n)
-       (let* ((shortname (citar--shorten-name n))
-              (pos (citar--shorten-name-position tnamelist n))
+       (let* ((name (funcall name-func n))
+              (pos (citar--render-name-position tnamelist n))
               (suffix
                (cond
                 ;; if last name in the list and we're truncating add et al.; otherwise, no suffix
@@ -1451,9 +1446,40 @@ replace last comma."
                  (concat " " andstr " "))
                 ;; otherwise, use a comma
                 (t ", "))))
-         (concat shortname suffix)))
+         (concat name suffix)))
      tnamelist "")))
 
+(defun citar--shorten-name (name)
+  "Return family NAME in `family, given' string.
+
+Otherwise, return as is."
+  (car (split-string name ", ")))
+
+(defun citar--shorten-names (namestr &optional truncate andstr)
+  "Return a list of family names from a list of full NAMESTR.
+To better accommodate corporate names, this will only shorten
+personal names of the form \"family, given\".
+
+With an integer TRUNCATE, will shorten the list, and ANDSTR will
+replace last comma."
+  (citar--render-names namestr #'citar--shorten-name truncate andstr))
+
+(defun citar--full-name (name)
+  "Return `given family' in `family, given' string NAME.
+
+Otherwise, return as is."
+  (let ((split-names (split-string name ", ")))
+    (if (> (length split-names) 1)
+        (concat (cadr split-names) " " (car split-names))
+      name)))
+
+(defun citar--full-names (namestr &optional truncate andstr)
+  "Return a list of full names from a list of full NAMESTR.
+
+With an integer TRUNCATE, will shorten the list, and ANDSTR will
+replace last comma."
+  (citar--render-names namestr #'citar--full-name truncate andstr))
+
 (defun citar--fields-for-format (template)
   "Return list of fields for TEMPLATE."
   (mapcan (lambda (fieldspec) (when (consp fieldspec) (cdr fieldspec)))