* Add the load-env command
load-env can be used to add environment variables dynamically via an
InputStream. This allows developers to create tools that output environment
variables as key-value pairs, then have the user load those variables in using
load-env. This supplants most of the need for an `eval` command, which is
mostly used in POSIX envs for setting env vars.
Fixes#3481
* fixup! Add the load-env command
