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_v3_10_5.cpp

@@ -0,0 +1,14 @@
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json_v3_10_5.hpp>
+using nlohmann::json;
+using nlohmann::ordered_json;
+
+TEST_CASE("use library v3.10.5 without inline namespace")
+{
+    json j;
+    j[ordered_json::json_pointer("/root")] = json::object();
+    // In v3.10.5 mixing json_pointers of different basic_json types
+    // results in implicit string conversion
+    CHECK(j.dump() == "{\"/root\":{}}");
+}