Make JSON_BRACE_INIT_COPY_SEMANTICS part of the ABI tag

The macro changes the body of the initializer-list constructor and adds a
to_json_tuple_impl overload, both with the same mangled names in either
mode, so mixing translation units silently picked one definition. Encode
it in the inline namespace as `_bics`, as JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
does with `_ldvcmp`. The macro is new in the unreleased 3.13.0, so no
existing namespace name changes.

- Move the macro's default into abi_macros.hpp so json_fwd.hpp computes
  the same namespace, and keep it defined under JSON_TEST_KEEP_MACROS.
- Check the tag in the ABI config tests and in the unit test.
- List `_bics` (and the missing `_dp`) in the namespace docs and in the
  natvis generator; regenerate nlohmann_json.natvis.
- Replace the "define it consistently" warning with an ABI note.

Suggested by @gregmarr in the review of #5544.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-brace-init-copy-semantics.cpp

@@ -9,8 +9,7 @@
 #include "doctest_compatibility.h"
 
 // This file tests the opt-in JSON_BRACE_INIT_COPY_SEMANTICS, so it defines the
-// macro itself rather than relying on a -D flag: the header #undefs the macro
-// before returning, so a test cannot check for it after the #include.
+// macro itself rather than relying on a -D flag, and runs in every build.
 #ifdef JSON_BRACE_INIT_COPY_SEMANTICS
     #undef JSON_BRACE_INIT_COPY_SEMANTICS
 #endif
@@ -28,8 +27,17 @@ using nlohmann::json;
 #include <utility>
 #include <vector>
 
+#define STRINGIZE_EX(x) #x
+#define STRINGIZE(x) STRINGIZE_EX(x)
+
 TEST_CASE("JSON_BRACE_INIT_COPY_SEMANTICS")
 {
+    SECTION("the macro is part of the ABI tag")
+    {
+        const std::string ns = STRINGIZE(NLOHMANN_JSON_NAMESPACE);
+        CHECK(ns.find("json_abi_bics") != std::string::npos);
+    }
+
     SECTION("single-element brace initialization copies the element (#5074)")
     {
         json const j_obj = {{"key", "value"}, {"num", 42}};