Vue's inbuilt dev server can be configured to proxy API for
development instead of directing developers to setup Nginx proxy.
Additionally this commit also adds an option to configure frontend port
on which dev server runs. Sample env config file is under
`frontend/.env.sample`, it can be copied to either `frontend/.env` or `frontend/.env.local`.
Available options are
LISTMONK_FRONTEND_PORT - Configure frontend dev server port (defaults to 8080)
LISTMONK_API_URL - Configure API server URL (defaults to http://127.0.0.1:9000)
Since env variables are sourced to current session we are prefixing it with `LISTMONK_`.
