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/string_escape.hpp

@@ -8,10 +8,9 @@
 
 #pragma once
 
-#include <nlohmann/detail/macro_scope.hpp>
+#include <nlohmann/detail/abi_macros.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
 
@@ -69,5 +68,5 @@ static void unescape(StringType& s)
     replace_substring(s, StringType{"~0"}, StringType{"~"});
 }
 
-} // namespace detail
-} // namespace nlohmann
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END