A Meson installation only provided the headers and a pkg-config file, so
consumers using find_package(nlohmann_json) could not find the library.
Meson now also installs nlohmann_jsonConfig.cmake and
nlohmann_jsonConfigVersion.cmake from the existing templates, plus a
static nlohmann_jsonTargets.cmake that defines the header-only imported
target with a relocatable include path.
A new CI job installs with Meson and builds tests/cmake_import/project
against the result.
Closes#3885
Signed-off-by: Niels Lohmann <mail@nlohmann.me>