Zephyr: Add action button to access `prj.conf`

Type: Feature Request

The file `prj.conf` that is in the root of each Zephry application is somewhat similar to cproject.yml.

Check if prj.conf exists, and add an action button "Open prj.conf file" to the West application name. 

<img width="848" height="140" alt="Image" src="https://github.com/user-attachments/assets/8b15414f-47f2-4848-979d-ce56e245bb5b" />
