feat(unstable/task): fail task on async command failure (#16301)

Tests and implementation are found here:
https://github.com/denoland/deno_task_shell/pull/59

This is a breaking change, but `deno task` is unstable.

> This changes async commands so that on non-zero exit code they will
fail the entire task. For example:
> 
> ```jsonc
> // task that asynchronously starts a server and starts a watcher for
the frontend
> "dev": "deno task server & deno task frontend:watch"
> ```
> 
> Previously when running `deno task dev`, if `deno task server` failed,
the entire command would not fail, which kept in line with `sh`, but
it's not very practical. This change causes `deno task dev` to fail.
> 
> To opt out, developers can add an `|| exit 0`:
> 
> ```jsonc
> "dev": "deno task server || exit 0 & deno task frontend:watch"
> ```

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