:meson: install headers + pkg-config

meson.build

@@ -11,3 +11,11 @@ nlohmann_json_dep = declare_dependency(
 nlohmann_json_multiple_headers = declare_dependency(
     include_directories: include_directories('include')
 )
+
+install_headers('single_include/nlohmann/json.hpp', subdir: 'nlohmann')
+
+pkgc = import('pkgconfig')
+pkgc.generate(name: 'nlohmann_json',
+    version: meson.project_version(),
+    description: 'JSON for Modern C++'
+)