feat: default to vim.snippet for snippet expansion if available (#1820)

doc/cmp.txt

@@ -85,6 +85,7 @@ A recommended configuration can be found below.
           -- require('luasnip').lsp_expand(args.body) -- For `luasnip` users.
           -- require'snippy'.expand_snippet(args.body) -- For `snippy` users.
           -- vim.fn["UltiSnips#Anon"](args.body) -- For `ultisnips` users.
+          -- vim.snippet.expand(args.body) -- For native neovim snippets (Neovim v0.10+)
         end,
       },
       window = {