fix: add draw-text method for raw 2D arrays

Application code (passepartout TUI) calls draw-text with a framebuffer
(2D array) as the first argument, but draw-text only had methods for
framebuffer-backend CLOS instances. Added a method on array that sets
cells directly on the framebuffer array, matching make-framebuffer's
return type.

org/backend-protocol.org

@@ -850,8 +850,9 @@ This means simple backends are always a "scroll and dump" mode —
 no cursor positioning, no escape sequences.
 
 #+BEGIN_SRC lisp :tangle ../src/backend/simple.lisp
-(defmethod draw-text ((b simple-backend) x y string fg bg
-                      &key bold italic underline reverse dim blink)
+ (defmethod draw-text ((b simple-backend) x y string fg bg
+                       &key bold italic underline reverse dim blink
+                       &allow-other-keys)
   (declare (ignore x y fg bg bold italic underline reverse dim blink))
   (backend-write b string))
 #+END_SRC