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

src/main/dummy/SDL_dummy_main.c

@@ -1,10 +1,7 @@
 /*
     SDL_dummy_main.c, placed in the public domain by Sam Lantinga  3/13/14
 */
-#include "../../SDL_internal.h"
-
-/* Include the SDL main definition header */
-#include "SDL_main.h"
+#include <SDL3/SDL.h>
 
 #ifdef main
 #undef main

src/main/gdk/SDL_gdk_main.c

@@ -18,14 +18,11 @@
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
 */
+#include <SDL3/SDL.h>
 
 /* Include this so we define UNICODE properly */
 #include "../../core/windows/SDL_windows.h"
 
-/* Include the SDL main definition header */
-#include "SDL.h"
-#include "SDL_main.h"
-
 #ifdef main
 #  undef main
 #endif /* main */

src/main/haiku/SDL_BApp.h

@@ -37,7 +37,6 @@ extern "C" {
 
 #include "../../SDL_internal.h"
 
-#include "SDL_video.h"
 
 /* Local includes */
 #include "../../events/SDL_events_c.h"

src/main/haiku/SDL_BeApp.cc

@@ -33,8 +33,6 @@
 
 #include "SDL_BApp.h"   /* SDL_BApp class definition */
 #include "SDL_BeApp.h"
-#include "SDL_timer.h"
-#include "SDL_error.h"
 
 #include "../../video/haiku/SDL_BWin.h"
 

src/main/n3ds/SDL_n3ds_main.c

@@ -18,12 +18,10 @@
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
 */
-
-#include "../../SDL_internal.h"
+#include <SDL3/SDL.h>
 
 #ifdef __3DS__
 
-#include "SDL_main.h"
 #include <3ds.h>
 
 #ifdef main

src/main/ngage/SDL_ngage_main.cpp

@@ -1,10 +1,7 @@
 /*
     SDL_ngage_main.c, originally for SDL 1.2 by Hannu Viitala
 */
-#include "../../SDL_internal.h"
-
-/* Include the SDL main definition header */
-#include "SDL_main.h"
+#include <SDL3/SDL.h>
 
 #include <e32std.h>
 #include <e32def.h>
@@ -16,7 +13,10 @@
 #include <w32std.h>
 #include <apgtask.h>
 
-#include "SDL_error.h"
+
+#ifdef main
+#undef main
+#endif
 
 extern "C" int main(int argc, char *argv[]);
 

src/main/ps2/SDL_ps2_main.c

@@ -1,14 +1,10 @@
 /*
     SDL_ps2_main.c, fjtrujy@gmail.com 
 */
-
-#include "SDL_platform.h"
+#include <SDL3/SDL.h>
 
 #ifdef __PS2__
 
-#include "SDL_main.h"
-#include "SDL_error.h"
-
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>

src/main/psp/SDL_psp_main.c

@@ -1,11 +1,10 @@
 /*
     SDL_psp_main.c, placed in the public domain by Sam Lantinga  3/13/14
 */
-#include "SDL_platform.h"
+#include <SDL3/SDL.h>
 
 #ifdef __PSP__
 
-#include "SDL_main.h"
 #include <pspkernel.h>
 #include <pspthreadman.h>
 

src/main/uikit/SDL_uikit_main.c

@@ -1,9 +1,7 @@
 /*
     SDL_uikit_main.c, placed in the public domain by Sam Lantinga  3/18/2019
 */
-
-/* Include the SDL main definition header */
-#include "SDL_main.h"
+#include <SDL3/SDL.h>
 
 #if defined(__IOS__) || defined(__TVOS__)
 

src/main/windows/SDL_windows_main.c

@@ -3,7 +3,7 @@
 
     The WinMain function -- calls your program's main() function
 */
-#include "SDL_platform.h"
+#include <SDL3/SDL.h>
 
 #ifdef __WIN32__
 
@@ -11,10 +11,6 @@
 #include "../../core/windows/SDL_windows.h"
 #include <shellapi.h> /* CommandLineToArgvW() */
 
-/* Include the SDL main definition header */
-#include "SDL.h"
-#include "SDL_main.h"
-
 #ifdef main
 #  undef main
 #endif /* main */

src/main/winrt/SDL_winrt_main_NonXAML.cpp

@@ -1,8 +1,8 @@
 /*
     SDL_winrt_main_NonXAML.cpp, placed in the public domain by David Ludwig  3/13/14
 */
+#include <SDL3/SDL.h>
 
-#include "SDL_main.h"
 #include <wrl.h>
 
 /* At least one file in any SDL/WinRT app appears to require compilation