Add "composer serve" script to start the webserver

[alexander-schranz]

The serve script would be start the webserver, like in other frameworks skeleton.

References in Yii and Mezzio Framework:

• https://github.com/mezzio/mezzio-skeleton/blob/4f60efe8a5d96b0178d446268c6fc2586f88df6b/composer.json#LL113C11-L117C11
• https://github.com/yiisoft/app/blob/264163b2af0d90217291a4e5977411173fe72add/composer.json#L12-L15