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_*

include/nlohmann/detail/json_pointer.hpp

@@ -27,8 +27,7 @@
 #include <nlohmann/detail/string_escape.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 
 /// @brief JSON Pointer defines a string syntax for identifying a specific value within a JSON document
 /// @sa https://json.nlohmann.me/api/json_pointer/
@@ -897,4 +896,5 @@ inline bool operator!=(json_pointer<RefStringTypeLhs> const& lhs,
 {
     return !(lhs == rhs);
 }
-}  // namespace nlohmann
+
+NLOHMANN_JSON_NAMESPACE_END