Emscripten support

[icculus]

Ideally I'd like the emscripten builds to just use CMake with emsdk, but it needs a little work in CMakeLists.txt and main.c ...

• The output file should be index.html (since this is meant to be a starting point, I'd rather a complete working thing is output instead of just the js/wasm files.
• Eventually this should just compile with -sUSE_SDL=3 but I assume there is no SDL3 package for Emscripten yet. Once we get to that point, I'd rather this starting app not have to bother with finding a copy of SDL3, but just use the included one.
• If main: Added _optional_ callback entry points. SDL#8247 lands, I'd like main.c to use SDL_MAIN_USE_CALLBACKS instead of SDL_main, so we don't have to stick in Emscripten #ifdefs, probably with a comment explaining that's an option but main() still works if they prefer.