feat: Kill all child processes when shell received CTRL-C

[mo]

Makes handling of these tasks nicer:

{
"tasks": {
"hashpipe-slow": "$(which cat) /dev/urandom | $(which head) -c 1000000000 | sha256sum",
"docker-compose": "docker compose -f compose.yaml up"
}
}

[CLAassistant]

All committers have signed the CLA.

[dsherret]

I'm not sure how this works because acquiring the lock in one place will block the other.

Instead of spawning a task, I feel like it should work to just add this to the select below? Something like:

      tokio::select! {
          result = child.wait() => /* ... */,
          _ = tokio::signal::ctrl_c() => {
              let _ = child.kill().await;
              // ...
          }
          _ = context.state.token().cancelled() => {
              let _ = child.kill().await;
              ExecuteResult::for_cancellation()
          }
      }

By the way, in the future when opening a PR, make sure to do it from a branch instead of the main branch. There's a couple reasons to do this:

1. It makes it easy to merge the latest main into your fork because your main branch is clean.
2. It allows maintaininers to push to your branch. Right now I get:

   To https://github.com/mo/deno_task_shell.git
    ! [remote rejected] mo/main -> mo/main (permission denied)
   error: failed to push some refs to 'https://github.com/mo/deno_task_shell.git'
   

[dsherret]

I kind of wonder if we should just add a single listener in Deno for ctrl+c then cancel the token on ShellState (we'd have to manually set it up to use https://docs.rs/deno_task_shell/0.18.1/deno_task_shell/fn.execute_with_pipes.html though).

[mo]

closing in favor of identical replacement PR:
#126