libobs: Add os_is_obs_plugin function

This function determines if something is an OBS plugin before attempting
to load it. On Windows, many plugins ship their dependent DLLs alongside
the plugin DLL, so OBS would load things like libcef.dll on startup only
to immediately free it. For other platforms, this is less of a concern
so this function is a no-op for now.

This improves startup time and reduces risk from dependent DLLs
potentially running code with unwanted side effects in DllMain.