feat: allow initalizing with "src" dir

[csvn]

I have existing v1 Fresh projects that I was planning to upgrade to v2. But I was unsure about how to migrate, since I was using a src/ folder. By reading the source code, I found that with Vite, the root property can be used to select the root folder for all code.

This PR re-introduces the docs example for how to use a "src" folder from Fresh v1, but also adds a --src-dir option during init such that a project can be initialized with a src dir.