std: Stabilize the `env` module

Now that the `std::env` module has had some time to bake this commit marks most
of its APIs as `#[stable]`. Some notable APIs that are **not** stable (and still
use the same `env` feature gate) are:

* `{set,get}_exit_status` - there are still questions about whether this is the
  right interface for setting/getting the exit status of a process.
* `page_size` - this may change location in the future or perhaps name as well.

This also effectively closes #22122 as the variants of `VarError` are
`#[stable]` now. (this is done intentionally)

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