Restructure inline namespace and allow version component to be disabled (#3683)

docs/examples/nlohmann_json_namespace.output

@@ -1 +1 @@
-nlohmann::json_v3_11_1
+nlohmann::json_abi_v3_11_1

docs/examples/nlohmann_json_namespace_no_version.cpp

@@ -0,0 +1,13 @@
+#include <iostream>
+
+#define NLOHMANN_JSON_NAMESPACE_NO_VERSION 1
+#include <nlohmann/json.hpp>
+
+// macro needed to output the NLOHMANN_JSON_NAMESPACE as string literal
+#define Q(x) #x
+#define QUOTE(x) Q(x)
+
+int main()
+{
+    std::cout << QUOTE(NLOHMANN_JSON_NAMESPACE) << std::endl;
+}

docs/examples/nlohmann_json_namespace_no_version.output

@@ -0,0 +1 @@
+nlohmann::json_abi