Allow def-env to hide environment variables

[kubouch]

Description

Fixes #907

> let-env FOO = "foo"

> def-env hide-foo [] {
    hide FOO
}

> $env.FOO
# Error, FOO not found.

Tests

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --all --all-features -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo build; cargo test --all --all-features to check that all the tests pass

[kubouch]

Please don't merge, I realized I can just insert callee's stack.env_vars to the caller's stack and not do all this shenanigans.

(I'm not sure how to convert to a draft PR on the phone.)

[kubouch]

Ah, nevermind, it was actually correct :-D