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/json_diagnostics.md

@@ -26,11 +26,16 @@ When the macro is not defined, the library will define it to its default value.
 
 ## Notes
 
-!!! danger "ABI incompatibility"
+!!! note "ABI compatibility"
 
-    As this macro changes the definition of the `basic_json` object, it MUST be defined in the same way globally, even
-    across different compilation units: `basic_json` objects with differently defined `JSON_DIAGNOSTICS` macros are
-    not compatible!
+    As of version 3.11.0, this macro is no longer required to be defined consistently throughout a codebase to avoid
+    One Definition Rule (ODR) violations, as the value of this macro is encoded in the namespace, resulting in distinct
+    symbol names. 
+    
+    This allows different parts of a codebase to use different versions or configurations of this library without
+    causing improper behavior.
+    
+    Where possible, it is still recommended that all code define this the same way for maximum interoperability.
 
 ## Examples
 
@@ -65,3 +70,4 @@ When the macro is not defined, the library will define it to its default value.
 ## Version history
 
 - Added in version 3.10.0.
+- As of version 3.11.0 the definition is allowed to vary between translation units.