feat: update post-start hooks for npm commands and add mailpit service

.ddev/config.yaml

@@ -16,7 +16,10 @@ web_environment:
 corepack_enable: false
 nodejs_version: "18"
 
+additional_services:
+  - mailpit
 hooks:
   post-start:
-    - exec: "npm install"
+    - exec-host: |
-    - exec: "npm run dev:css"
+        ddev npm install
+        ddev npm run dev:css -- --watch &