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_*

docs/mkdocs/docs/api/macros/index.md

@@ -27,6 +27,11 @@ header. See also the [macro overview page](../../features/macros.md).
 - [**JSON_SKIP_LIBRARY_VERSION_CHECK**](json_skip_library_version_check.md) - skip library version check
 - [**NLOHMANN_JSON_VERSION_MAJOR**<br>**NLOHMANN_JSON_VERSION_MINOR**<br>**NLOHMANN_JSON_VERSION_PATCH**](nlohmann_json_version_major.md) - library version information
 
+## Library namespace
+
+- [**NLOHMANN_JSON_NAMESPACE**](nlohmann_json_namespace.md) - full name of the `nlohmann` namespace
+- [**NLOHMANN_JSON_NAMESPACE_BEGIN**<br>**NLOHMANN_JSON_NAMESPACE_END**](nlohmann_json_namespace_begin.md) - open and close the library namespace
+
 ## Type conversions
 
 - [**JSON_DISABLE_ENUM_SERIALIZATION**](json_disable_enum_serialization.md) - switch off default serialization/deserialization functions for enums