Add external reader support in pkl-gradle

[HT154]

[bioball]

@HT154 can we add some test coverage for this?

I think this would involve moving TestExternalReaderProcess to pkl-commons-test, and adding a corresponding test to org.pkl.gradle.EvaluatorsTest.

Hint: the Groovy syntax for maps is [key: value]. External readers would look something like:

externalModuleReaders = ["foo": new org.pkl.gradle.ExternalReader("bar")]

[HT154]

It looks like this hangs right now for reasons I'm not understanding. It's intermittent on *nix systems and seemingly consistent on Windows (but I don't have a system to test that locally). Will likely need a hand debugging this.