Add ‘paths.relative_path’ function

[mrkkrp]

This is a function that we found ourselves needing sometimes.

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[mboes]

Related: https://github.com/bazelbuild/bazel-skylib/issues/30.

[aiuto]

This seems to have gone stale? Is it going to move forward or should we close it.

[tetromino]

Is there a chance of this getting reviewed and merged? I just wrote an almost identical function in starlark for some rules this afternoon because I didn't find it in Skylib - would have been nice if it were available :)