filesystem: Add a file watcher API

[icculus]

And @TylerGlaiel mentioned this:

Random request if filesystem stuff is getting added into SDL, tracking when a file changes (for hot-reloading purposes) is something that requires platform-specific code (or weird hacks) to achieve normally, so some stuff in SDL to help with that would be pretty cool actually

SDL_WatchFileForChanges(const char *path);

and then some SDL_EVENT_FILECHANGED to get notified when the file is written

Originally posted by @icculus in #6644

Just moving this to its own bug for now so we can close the very old and scattered SDL3 filesystem API megathread. I'm tossing this into 3.6.0 for near-future consideration, but I have like six other fires to put out before I can actually work on this, so it might slip from that milestone.

[slouken]

For an example of a potential Linux implementation, check out LINUX_InotifyJoystickDetect() in src/joystick/linux/SDL_sysjoystick.c

[slouken]

Also, SDL_steam_virtual_gamepad.c would directly benefit from something like this.