Do not use Qt's keyword macros as identifiers

The prefix tree used by unflatten() (#5836) had a member named slots, which
Qt defines as an empty macro, so including a Qt header before this library
failed to compile. Rename it to table and add a test that includes the
library with Qt's keyword macros (slots, signals, emit, foreach, forever)
defined.

Fixes #5865

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

docs/mkdocs/docs/community/quality_assurance.md

@@ -135,6 +135,9 @@ Note: Some modern features (like C++20 ranges or filesystem support) may be disa
   headers are included.
 - [x] On Windows, the library is compiled with `<Windows.h>` being included to detect and avoid common bugs (see
   [`unit-windows_h.cpp`](https://github.com/nlohmann/json/blob/develop/tests/src/unit-windows_h.cpp)).
+- [x] The library is compiled with Qt's keyword macros (`slots`, `signals`, `emit`, `foreach`, `forever`) being defined
+  to make sure it does not use them as identifiers (see
+  [`unit-qt_keywords.cpp`](https://github.com/nlohmann/json/blob/develop/tests/src/unit-qt_keywords.cpp)).
 - [x] The library is compiled with exceptions disabled to support alternative means of error handling.
 
 ## Stable public API