fhir: map reference in extension

[hunterachieng]

Summary

• Added a reference mapper in mapExtension in generate-code.
• Added a failing test for a created extension with correctly mapped reference. (This should fail and pass after fhir-eswatini is regenerated)

Fixes #1583

Details

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[hunterachieng]

@josephjclark I had branched off of the previous branch, hence the many commit messages

[josephjclark]

Fantastico!

[josephjclark]

@hunterachieng couple of notes for future reference

1. When we make a change to the generator, we need to a) bump the version (either with changeset or by manually bumping the package.json and changset), and b) when the generator version changes, re-run the builder so that the package.json metadata is correct (check the package.json for fhir-eswatini on this branch now that I've versioned it)
2. Just in case it's useful later, in the build/mappings.js file in the IG folder, you can specifiy includes with a resource type (like Patient) to ONLY re-generate that resource. I do this in dev all the time - super helpful