feat(view): add api get_selected_index (#1986)

lua/cmp/init.lua

@@ -88,6 +88,12 @@ cmp.visible = cmp.sync(function()
   return cmp.core.view:visible() or vim.fn.pumvisible() == 1
 end)
 
+---Get what number candidates are currently selected.
+---If not selected, nil is returned.
+cmp.get_selected_index = cmp.sync(function()
+  return cmp.core.view:get_selected_index()
+end)
+
 ---Get current selected entry or nil
 cmp.get_selected_entry = cmp.sync(function()
   return cmp.core.view:get_selected_entry()