Switch header convention from #include "SDL.h" to #include <SDL3/SDLh>

I ran this script in the include directory:
```sh
sed -i '' -e 's,#include "\(SDL.*\)",#include <SDL3/\1>,' *.h
```

I ran this script in the src directory:
```sh
for i in ../include/SDL3/SDL*.h
do hdr=$(basename $i)
   if [ x"$(echo $hdr | egrep 'SDL_main|SDL_name|SDL_test|SDL_syswm|SDL_opengl|SDL_egl|SDL_vulkan')" != x ]; then
        find . -type f -exec sed -i '' -e 's,#include "\('$hdr'\)",#include <SDL3/\1>,' {} \;
    else
        find . -type f -exec sed -i '' -e '/#include "'$hdr'"/d' {} \;
    fi
done
```

Fixes https://github.com/libsdl-org/SDL/issues/6575

WhatsNew.txt

@@ -6,6 +6,7 @@ This is a list of major changes in SDL's version history.
 ---------------------------------------------------------------------------
 
 General:
+* SDL headers should now be included as `#include <SDL3/SDL.h>`
 * M_PI is no longer defined in SDL_stdinc.h, now the symbols SDL_PI_D (double) and SDL_PI_F (float) are available
 * SDL_GetWindowWMInfo() returns a standard int result code instead of SDL_bool, and takes SDL_SYSWM_CURRENT_VERSION as a new third parameter
 * The preprocessor symbol __MACOSX__ has been renamed __MACOS__