Enable users to define their env var validations

[infomiho]

Caveats:

• users will need to be careful not to cause circular imports. Ideally they use a file only for env schemas e.g. env.ts
• user will need to use the EnvValidationFn with satisfies
• maybe this will clash with Build time validation for client env #2392

[Martinsos]

Caveats:

• users will need to be careful not to cause circular imports. Ideally they use a file only for env schemas e.g. env.ts
• user will need to use the EnvValidationFn with satisfies
• maybe this will clash with Build time validation for client env #2392

@infomiho could you please describe how this works, what is the experience for the user? I see there are no docs yet, and it would help me with reviewing so I don't have to guess it from the code + so I can consider if code does indeed do what the idea was.