Export layer_env::PATH_LIST_SEPARATOR

[edmorley]

Currently when setting ModificationBehavior::Delimiter in a buildpack (eg for a path based env var), one has to either:

1. Hardcode : as the delimeter in the buildpack (and just ignore the fact it wouldn't work on Windows)
2. Reimplement the platform specific definitions here:

   libcnb.rs/libcnb/src/layer_env.rs

   Lines 921 to 925 in dadd5b5

   	#[cfg(target_family = "unix")]
   	const PATH_LIST_SEPARATOR: &str = ":";
   	#[cfg(target_family = "windows")]
   	const PATH_LIST_SEPARATOR: &str = ";";

We should export them instead.

Also, those constants should probably be moved higher up in that file, since currently they are hidden after the tests.

[Malax]

I like this idea! 👍🏻

[edmorley]

I think we should maybe focus on fixing the default behaviour upstream instead of this - see:
buildpacks/spec#285