os: add a mechanism for signal handling

There is currently no way to handle system-signals in ASPL without custom implementation calls. I think this feature should be part of the standard library.

Example usage:
```aspl
import os

os.register_signal_handler(Signal.Interrupt, callback(){
print("SIGINT detected...")
// TODO: Do something here
}
```

No implementation files or tests are named. Start by locating the standard-library and runtime entry points for OS integration, then determine how signal registration should map to ASPL callbacks and what tests can demonstrate handling SIGINT.

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