Add mode flag

[Lzadhito]

Hi, I've been wondering if we could manage adding a mode flag like we do in vite such as vite build --mode development.
https://vitejs.dev/guide/env-and-mode#modes

By the way, thank you very much for the hardwork. I've been using it for a 2-3 months on solidstart and I could said its pretty solid :)

[LexSwed]

Upvote. Applications might require different runtimes for acceptance testing or staging environment.