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/nlohmann_json_namespace.md

@@ -0,0 +1,26 @@
+# NLOHMANN_JSON_NAMESPACE
+
+```cpp
+#define NLOHMANN_JSON_NAMESPACE
+```
+
+This macro evaluates to the full name of the `nlohmann` namespace, including
+the name of a versioned and ABI-tagged inline namespace. Use this macro to
+unambiguously refer to the `nlohmann` namespace.
+
+## Default definition
+
+The default value consists of a prefix, a version string, and optional ABI tags
+depending on whether ABI-affecting macros are defined (e.g.,
+[`JSON_DIAGNOSTICS`](json_diagnostics.md), and
+[`JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`](json_use_legacy_discarded_value_comparison.md)).
+
+When the macro is not defined, the library will define it to its default value.
+
+## See also
+
+- [`NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END`](nlohmann_json_namespace_begin.md)
+
+## Version history
+
+- Added in version 3.11.0.