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

test/testutils.h

@@ -7,7 +7,7 @@ SPDX-License-Identifier: Zlib
 #ifndef TESTUTILS_H
 #define TESTUTILS_H
 
-#include "SDL.h"
+#include <SDL3/SDL.h>
 
 SDL_Texture *LoadTexture(SDL_Renderer *renderer, const char *file, SDL_bool transparent,
                          int *width_out, int *height_out);