Feature: get environment variables necessary to resolve runfiles

[liningpan]

It would be helpful to add a method to the runfiles library that returns a set of environment variables for sub-processes to resolve runfiles.

For reference, the Python runfiles library provides this functionality.

https://github.com/bazelbuild/rules_python/blob/e86252ffd6d1a1bf32ae99933acc5ab49b78ec1e/python/runfiles/runfiles.py#L200

[DavidZbarsky-at]

I am currently poking at runfiles library, I can add this