add rnix, a nix lsp server

lua/lsp.lua

@@ -59,6 +59,7 @@ local servers = {
   "ocamllsp",
   "vimls",
   "clangd",
+  "rnix",
 }
 for _, lsp in ipairs(servers) do
     nvim_lsp[lsp].setup {capabilities = capabilities, on_attach = on_attach}