feat: add null-ls

lua/custom/plugins/null-ls.lua

@@ -0,0 +1,13 @@
+return {
+  "nvimtools/none-ls.nvim",
+  config = function()
+    local null_ls = require("null-ls")
+
+    null_ls.setup({
+      sources = {
+        null_ls.builtins.formatting.prettier,
+        null_ls.builtins.diagnostics.eslint,
+      },
+    })
+  end
+}