feat: add conformance test suite

[crowecawcaw]

This PR proposes a language/implementation agnostic test suite that allows developers to confidently author conforming OJD libraries. The major components are:

• A README with a high level description of the suite. Recommend to start here. Rendered markdown: https://github.com/crowecawcaw/openjd-specifications/tree/conformance/conformance-tests
• An example test runner that runs the test data
• A bunch of test cases created from inspecting the spec and translated from the openjd-model package.

The openjd-model packages passes most but not all tests. See the list of failing tests at the top of the example test runner.

As part of assembling this test suite, I opened an issue to clarify the spec where it was implicit or where the model library seemed to differ: #100

Some choices made along the way:

• Use YAML for test data because it's easy to read and most languages have a good parser for it. (JSON was considered but it does not allow comments.)
• Organize test data into directories based on test type.
• Make a separate directory for each extension. Makes it easy to find the new test cases for a new feature and easy to ignore extensions that are not yet supported.

High level feedback welcome!

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