Does not work in C++23 due to conflicts with import std cxx modules

**Describe the bug**
In past discussions, Catch2 devs explained that it's not possible to migrate the project for CXX Modules, because modules cannot export macros, which is correct (https://github.com/catchorg/Catch2/issues/2299). However, even if the user has cxx modules in its code, specially C++23 import std, then it cannot currently interoperate with header only catch2, because it imposes its #include , generating ODR with import std, if compiler is not "smart enough" (which is currently the case and maybe will never be fully solved).
My problem now is that I'm a long time Catch2 user, and I cannot make it work with Clang 19 or GCC 15, because I'm using `import std` on C++23 and that's incompatible with current Catch2 design (but it's easy to solve!).

**Expected behavior**
Catch2 should provide some way to disable its #include for STD things, for example, adding an option or macro like "CATCH2_DISABLE_STD_INCLUDE"... so , all #include should be surrounded by this option:

```
#ifndef CATCH2_DISABLE_STD_INCLUDE
#include
#include
#endif
```

Macro name can also be `CATCH_USE_STDLIB_MODULE`, as in, https://github.com/catchorg/Catch2/issues/2575, but I don't know if Catch2 is interested in really using import std... maybe not! But this proposal here is simpler, it just allows users to choose between its own `import std` or the current Catch2 #include dependencies.

**Reproduction steps**
Create some Module.cpp file, with import std, and try to #include catch2, or try to link against it. ODR violations will be reported.

**Platform information:**
- OS: Ubuntu Linux
- Compiler: Clang 19 or GCC 15

**Additional context**
If necessary, I can do the PR... specially, for the amalgamated hpp file, it would be nice to disable all its header inclusions.