fix(cmp): bring back default keybindings (#2470)

lua/lvim/core/cmp.lua

@@ -248,7 +248,7 @@ M.config = function()
       { name = "treesitter" },
       { name = "crates" },
     },
-    mapping = {
+    mapping = cmp.mapping.preset.insert {
       ["<C-k>"] = cmp.mapping.select_prev_item(),
       ["<C-j>"] = cmp.mapping.select_next_item(),
       ["<C-d>"] = cmp.mapping.scroll_docs(-4),