Use z.object for the env vars

[vdawg-git]

Currently you use a simple object with the env variable names as keys.
Like this we cannot use transform and refine functionality of Zod.

My use case:
I got a BETTER_AUTH_URL which is also the BASE_URL. Ideally I just transform BETTER_AUTH_URL to also be BASE_URL.

Also .refine would be useful if some env needs A and B if C is set.