chore(core): remove jsonpp 🔱

[mcdurdin]

jsonpp was being used only in a single unit test, and so this was a lot of overhead and complexity for very little benefit. We may want to add some more unit tests in the future for state_api.tests.cpp to do a deep comparison of the state?

Also: move keyboard_api and options_api tests to gtest - working on this precipitated the jsonpp removal.

Test-bot: skip

[keymanapp-test-bot]

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