feat(layout): register footnote frames with the parallel package

Registered two additional frames using the parallel package for accommodate footnotes. These frames are set up to align with the main parallel frames and maintain a consistent layout for footnotes (fixed height).

classes/diglot.lua

@@ -49,6 +49,10 @@ function class:_init (options)
          left = "a",
          right = "b",
       },
+		ftn_frames = {
+			ftn_left = "c",
+			ftn_right = "d",
+		},
    })
 end