environment types

Sveltekit has this cool feature where you use `$env.foo` and it ensures that it exists at compile time. That would be useful in python.

No files, tests, or entry points are identified in the issue. First define the intended Python environment-variable typing behavior and how it should compare with SvelteKit’s compile-time guarantee, then locate the relevant type-checker entry points and add tests that establish what counts as done.

Written by the indexing model from the issue text.