fix: Restore some keybindings to nvim-cmp

In recent versions, nvim-cmp plugin has dropped the default keybindings
that had to use the plugin, enforcing users to set their own.

nvim/lsp_client_settings.vim

@@ -53,7 +53,13 @@ local on_attach = function(client, bufnr)
       virtual_text = false
     }
   )
-  require("cmp").setup {
+
+  local cmp = require("cmp")
+  cmp.setup {
+    mapping = {
+      ["<C-n>"] = cmp.mapping.select_next_item { behavior = cmp.SelectBehavior.Insert },
+      ["<C-p>"] = cmp.mapping.select_prev_item { behavior = cmp.SelectBehavior.Insert },
+    },
     formatting = {format = cmp_format},
     sources = {
       {name = "nvim_lsp"},