Fix backspace keystroke not working for multi-editing

Fix notepad-plus-plus#14359 (comment)

PowerEditor/src/ScintillaComponent/ScintillaEditView.cpp

@@ -581,6 +581,7 @@ LRESULT ScintillaEditView::scintillaNew_Proc(HWND hwnd, UINT Message, WPARAM wPa
 					case VK_HOME:
 					case VK_END:
 					case VK_RETURN:
+					case VK_BACK:
 						execute(SCI_SETSELECTIONMODE, SC_SEL_STREAM); // When it's rectangular selection and the arrow keys are pressed, we switch the mode for having multiple carets.
 
 						execute(SCI_SETSELECTIONMODE, SC_SEL_STREAM); // the 2nd call for removing the unwanted selection while moving carets.