Allow type safe access to env (+ show nice errors on .env not set)

[nikitavoloboev]

There is Discord thread regarding this feature request too but it got forgotten about thus opening discussion/issue here.

Essentially it would be great to have type safe access to the env. Similar to what Astro is doing and perhaps t3-env.

This is prompted also by me receiving an error like this (which was hard to debug):

Because this was not set:

After I set the env in .env, things worked, I hope that can be improved/fixed too.

[SeanCassiere]

Haven't tested this yet, but we'll likely need to get the basic's from SolidStart's docs into TSR's pages for Start.

https://docs.solidjs.com/configuration/environment-variables