add nix ftplugin

lua/plugins/conform.lua

@@ -0,0 +1,11 @@
+return {
+    'stevearc/conform.nvim',
+    opts = {
+        formatters_by_ft = {
+            lua = { "stylua" },
+            python = { "isort", "black" },
+            rust = { "rustfmt", lsp_format = "fallback" },
+            javascript = { "prettierd", "prettier", stop_after_first = true },
+        },
+    },
+}