add implementation of viK, completing hjkl movement keys

fcore/app-update.sml

@@ -69,10 +69,27 @@ struct
       (newApp, drawMsg)
     end
 
+  fun moveUp (app: app_type) =
+    let
+      val {buffer, windowWidth, windowHeight, startLine, cursorIdx, ...} = app
+
+      val buffer = LineGap.goToIdx (cursorIdx, buffer)
+      val cursorIdx = Cursor.viK (buffer, cursorIdx)
+
+      val buffer = LineGap.goToLine (startLine, buffer)
+      val drawMsg = TextBuilder.build
+        (startLine, cursorIdx, buffer, windowWidth, windowHeight)
+
+      val newApp = AppWith.bufferAndCursorIdx (app, buffer, cursorIdx)
+    in
+      (newApp, drawMsg)
+    end
+
   fun handleChr (app: app_type, chr) =
     case chr of
       #"h" => moveLeft app
     | #"j" => moveDown app
+    | #"k" => moveUp app
     | #"l" => moveRight app
     | _ => (app, [])