feat(runtime): add Deno.addSignalListener API

[kt3k]

This PR implements Deno.addSignalListener and Deno.removeSignalListener APIs as disucessed in #11158

The basic usage:

const listener = () => {
  console.log("SIGTERM!")
};
// Starts listening to SIGTERM
Deno.addSignalListener("SIGTERM", listener);
...
// Stops listening to SIGTERM
Deno.removeSignalListener("SIGTERM", listener);

closes #11158

[piscisaureus]

Where does BadResource get thrown?
I think it could only happen if there is an error in the listener callback, in which case we should not be handling it.

[kt3k]

You are right. This handling was unnecessary. Updated.

[piscisaureus]

LGTM

[bartlomieju]

LGTM too, I like this API better; it will be useful for std/node

[bartlomieju]

BTW, should we also stabilize it in 1.16?

[bartlomieju]

@kt3k please land ASAP as this PR will unblock polyfilling more process.on() events in std/node.

[piscisaureus]

BTW, should we also stabilize it in 1.16?

Let's get some experience with it before we decide.
I have some concern about what happens when you attach an async function as a listener.
If a signal is fired multiple times (e.g. press ^C twice) you might end up with the same cleanup process running at the same time.