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

include/SDL3/SDL_loadso.h

@@ -0,0 +1,115 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2022 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+/**
+ *  \file SDL_loadso.h
+ *
+ *  System dependent library loading routines
+ *
+ *  Some things to keep in mind:
+ *  \li These functions only work on C function names.  Other languages may
+ *      have name mangling and intrinsic language support that varies from
+ *      compiler to compiler.
+ *  \li Make sure you declare your function pointers with the same calling
+ *      convention as the actual library function.  Your code will crash
+ *      mysteriously if you do not do this.
+ *  \li Avoid namespace collisions.  If you load a symbol from the library,
+ *      it is not defined whether or not it goes into the global symbol
+ *      namespace for the application.  If it does and it conflicts with
+ *      symbols in your code or other shared libraries, you will not get
+ *      the results you expect. :)
+ */
+
+#ifndef SDL_loadso_h_
+#define SDL_loadso_h_
+
+#include <SDL3/SDL_stdinc.h>
+#include <SDL3/SDL_error.h>
+
+#include <SDL3/begin_code.h>
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Dynamically load a shared object.
+ *
+ * \param sofile a system-dependent name of the object file
+ * \returns an opaque pointer to the object handle or NULL if there was an
+ *          error; call SDL_GetError() for more information.
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_LoadFunction
+ * \sa SDL_UnloadObject
+ */
+extern DECLSPEC void *SDLCALL SDL_LoadObject(const char *sofile);
+
+/**
+ * Look up the address of the named function in a shared object.
+ *
+ * This function pointer is no longer valid after calling SDL_UnloadObject().
+ *
+ * This function can only look up C function names. Other languages may have
+ * name mangling and intrinsic language support that varies from compiler to
+ * compiler.
+ *
+ * Make sure you declare your function pointers with the same calling
+ * convention as the actual library function. Your code will crash
+ * mysteriously if you do not do this.
+ *
+ * If the requested function doesn't exist, NULL is returned.
+ *
+ * \param handle a valid shared object handle returned by SDL_LoadObject()
+ * \param name the name of the function to look up
+ * \returns a pointer to the function or NULL if there was an error; call
+ *          SDL_GetError() for more information.
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_LoadObject
+ * \sa SDL_UnloadObject
+ */
+extern DECLSPEC void *SDLCALL SDL_LoadFunction(void *handle,
+                                               const char *name);
+
+/**
+ * Unload a shared object from memory.
+ *
+ * \param handle a valid shared object handle returned by SDL_LoadObject()
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_LoadFunction
+ * \sa SDL_LoadObject
+ */
+extern DECLSPEC void SDLCALL SDL_UnloadObject(void *handle);
+
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+#include <SDL3/close_code.h>
+
+#endif /* SDL_loadso_h_ */
+
+/* vi: set ts=4 sw=4 expandtab: */