feat(unstable): clean environmental variables for subprocess

[crowlKats]

Closes #3981

[ry]

Why not Deno.env.clear() to stick with the Map-like interface?

I'm a little concerned that this is immediately going to stable. Is it possible to make this unstable first?

[bartlomieju]

Why not Deno.env.clear() to stick with the Map-like interface?

This is not API for Deno.env, it's an option when starting a subprocess with Deno.run(). But I agree with @bnoordhuis suggestion that it might be better to name it clearEnv

[crowlKats]

Is it possible to make this unstable first?

sure, i am quite in favour of that.

[bartlomieju]

LGTM, thanks @crowlKats!