Distribution, packaging

This is more of a TODO list to make the project distributable:

- [ ] Have a package for every dependency:
- [x] `double-conversion` (https://repology.org/project/double-conversion/versions)
- [ ] `jtckdint` (https://repology.org/projects/?search=jtckdint)
Vendored source diverges from upstream since https://github.com/jart/json.cpp/commit/60a4a04a55c68e3d90b6364720d96dbace44fab0
- [ ] Decide on a binary artifact/package name (`jt-json-cpp`? `jsoncpp` is "taken" https://vcpkg.link/ports/jsoncpp)
- [ ] Decide on static-only or static/shared (requires export macros)
- [ ] CMake (optional, but would make cross-platform easier and I volunteer to make a PR)

Start with the checklist in this issue, especially the jtckdint dependency and its divergence from upstream. Resolve the binary artifact name and static-only versus static/shared distribution decisions, then consider the proposed CMake support. Done means the project has a defined distributable packaging approach covering these items.

Written by the indexing model from the issue text.