Make WinRT's SDL_main header-only

As the implementation requires C++, the user will have to include
SDL_main.h in a C++ source file (that needs to be compiled with /ZW).
It's ok to keep the standard main() implementation in plain C and use
an otherwise empty C++ source file for the SDL_main implementation part,
if both source files #include <SDL3/SDL_main.h>

Including SDL_main.h in a C source file will print a message at
compilation (when building for WinRT or possibly other not yet implemented
platforms that require C++ for main), to remind the user of also
including it in a .cpp source file. This message/warning can be disabled
with #define SDL_MAIN_NOIMPL before including SDL_main.h in the C file.
When including it in a .cpp file, there will be a compiler error with
helpful message if it's not compiled with /ZW

For this I renamend _SDL_MAIN_NOIMPL to SDL_MAIN_NOIMPL, because now it's
not for internal use only anymore, but also useful for users (that want
their main() function in a different file than the SDL_main implementation)

Add a project for the testdraw2.c test to the WinRT solution to at least
get some minimal testing on WinRT.
I won't add all tests because it's a lot of manual clicking per test,
but this should be better than nothing :)

test/main.cpp

@@ -0,0 +1,11 @@
+/*
+ * This file is supposed to be used to build tests on platforms that require
+ * the main function to be implemented in C++, which means that SDL_main's
+ * implementation needs C++ and thus can't be included in test*.c
+ *
+ * Placed in the public domain by Daniel Gibson, 2022-12-12
+ */
+
+#include <SDL3/SDL_main.h>
+
+// that's all, folks!