Severel changes.

.config/nvim/after/plugin/null-ls.lua

@@ -0,0 +1,12 @@
+local ok, nls = pcall(require, 'null-ls')
+
+if not ok then
+  return
+end
+
+nls.setup({
+    sources = {
+        nls.builtins.formatting.stylua,
+        nls.builtins.diagnostics.credo,
+    },
+})