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/core/windows/SDL_immdevice.h

@@ -22,8 +22,6 @@
 #ifndef SDL_IMMDEVICE_H
 #define SDL_IMMDEVICE_H
 
-#include "SDL_atomic.h"
-#include "SDL_audio.h"
 
 #define COBJMACROS
 #include <mmdeviceapi.h>

src/core/windows/SDL_windows.c

@@ -23,7 +23,6 @@
 #if defined(__WIN32__) || defined(__WINRT__) || defined(__GDK__)
 
 #include "SDL_windows.h"
-#include "SDL_error.h"
 
 #include <objbase.h>    /* for CoInitialize/CoUninitialize (Win32 only) */
 #if defined(HAVE_ROAPI_H)

src/core/windows/SDL_windows.h

@@ -89,8 +89,6 @@
 #endif
 #endif
 
-#include "SDL_rect.h"
-
 /* Routines to convert from UTF8 to native Windows text */
 #define WIN_StringToUTF8W(S) SDL_iconv_string("UTF-8", "UTF-16LE", (const char *)(S), (SDL_wcslen(S)+1)*sizeof(WCHAR))
 #define WIN_UTF8ToStringW(S) (WCHAR *)SDL_iconv_string("UTF-16LE", "UTF-8", (const char *)(S), SDL_strlen(S)+1)