reimplement 'dt' motion

fcore/normal-mode/make-normal-delete.sml

@@ -740,6 +740,21 @@ struct
         end
     end
 
+  fun deleteTillNextChr (app: app_type, count, chr, time) =
+    let
+      val {buffer, cursorIdx, ...} = app
+      val buffer = LineGap.goToIdx (cursorIdx, buffer)
+      val newCursorIdx =
+        Cursor.toNextChrNew (buffer, cursorIdx, {findChr = chr, count = count})
+    in
+      if newCursorIdx = ~1 then
+        NormalFinish.clearMode app
+      else
+        let val length = newCursorIdx - cursorIdx
+        in deleteAndFinish (app, cursorIdx, length, buffer, time)
+        end
+    end
+
   fun deleteToStart (app: app_type, time) : AppType.app_type =
     let
       val {cursorIdx, buffer, windowWidth, windowHeight, dfa, ...} = app

fcore/normal-mode/normal-mode.sml

@@ -298,9 +298,7 @@ struct
       else
         (* have to continue parsing string *)
         case String.sub (str, strPos + 1) of
-          #"t" =>
-            NormalDelete.deleteToChr
-              (app, 1, Cursor.tillNextChr, op+, chrCmd, time)
+          #"t" => NormalDelete.deleteTillNextChr (app, count, chrCmd, time)
         | #"T" =>
             NormalDelete.deleteToChr
               (app, 1, Cursor.tillPrevChr, op-, chrCmd, time)