feature(proposal): Bindings for system environment variable

[pmbanugo]

At the moment I have to put the values for environment variables as key bindings. I'd like a way to read system environment variables similar to process.env in Node.js. Perhaps one way to make it work could be explicitly binding to a system environment variable. Another option could be specifying it via the runtime/cli.

[kentonv]

I think it would make sense to allow a text binding to take its value from an environment variable, and this should be easy to implement.

So like:

binding = (name = "bindingName", (fromEnvironment = "VAR_NAME"))

It's important that this be declared explicitly, though, so that secrets from the environment don't accidentally leak to workers.