chore(core): rename json.hpp to jsonpp.hpp 🛩

[mcdurdin]

The name json.hpp conflicts with the JSON for Modern C++ library that is also used by Developer. Renames json.hpp and json.cpp to jsonpp.hpp and jsonpp.cpp aka "JSON Pretty Printer".

@keymanapp-test-bot skip

[keymanapp-test-bot[bot]]

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[keyman-server]

Changes in this pull request will be available for download in Keyman version 16.0.54-alpha