Add versioned, ABI-tagged inline namespace and namespace macros (#3590)

* Add versioned inline namespace

Add a versioned inline namespace to prevent ABI issues when linking code
using multiple library versions.

* Add namespace macros

* Encode ABI information in inline namespace

Add _diag suffix to inline namespace if JSON_DIAGNOSTICS is enabled, and
_ldvcmp suffix if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON is enabled.

* Move ABI-affecting macros into abi_macros.hpp

* Move std_fs namespace definition into std_fs.hpp

* Remove std_fs namespace from unit test

* Format more files in tests directory

* Add unit tests

* Update documentation

* Fix GDB pretty printer

* fixup! Add namespace macros

* Derive ABI prefix from NLOHMANN_JSON_VERSION_*

tests/abi/inline_ns/use_current.cpp

@@ -0,0 +1,28 @@
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+
+TEST_CASE("use current library with inline namespace")
+{
+    SECTION("implicitly")
+    {
+        using nlohmann::json;
+        using nlohmann::ordered_json;
+
+        json j;
+        // In v3.10.5 mixing json_pointers of different basic_json types
+        // results in implicit string conversion
+        j[ordered_json::json_pointer("/root")] = json::object();
+        CHECK(j.dump() == "{\"root\":{}}");
+    }
+
+    SECTION("explicitly")
+    {
+        using NLOHMANN_JSON_NAMESPACE::json;
+        using NLOHMANN_JSON_NAMESPACE::ordered_json;
+
+        json j;
+        j[ordered_json::json_pointer("/root")] = json::object();
+        CHECK(j.dump() == "{\"root\":{}}");
+    }
+}