Add support for Environment vars in RunConfig

[gzm0]

This is the first step towards scala-js/scala-js#4686.

[sjrd]

That was fast ^^

I have only one comment below.

Also: there's a typo in the second commit message: "Enviornment"

[sjrd]

I find it confusing that this method is called withAdditionalEnv, but it sets instead of adding. It's even more confusing that the method below actually adds to the map.

Have you considered calling it env instead of additionalEnv, hence this method would be withEnv, and the one below would remain withAdditionalEnv?

I understand the env here is in addition to the ExternalJSRun.Config.env. But that one is anyway already in addition to the host process environment vars. It doesn't seem problematic to me to keep env at each level.

[gzm0]

I have renamed it to env and removed the helper. While it might be convenient every once in a while, it is likely that at the call-site, the env is just forwarded anyways.

So it is definitely not worth the confusion.

[sjrd]

Is there anything else we should do here, or should I publish a new version now?

[gzm0]

I think the next step is to publish a new version indeed.