Add amalgamated json-fwd.hpp to release (#3687)

* 🔧 add amalgamated json-fwd.hpp to release

* 🔧 add amalgamated json-fwd.hpp to package managers

meson.build

@@ -14,6 +14,7 @@ nlohmann_json_multiple_headers = declare_dependency(
 
 if not meson.is_subproject()
 install_headers('single_include/nlohmann/json.hpp', subdir: 'nlohmann')
+install_headers('single_include/nlohmann/json_fwd.hpp', subdir: 'nlohmann')
 
 pkgc = import('pkgconfig')
 pkgc.generate(name: 'nlohmann_json',