Allow arrow functions, change callable parameter definitions to callable():mixed

[gisostallenberg]

I know the callable parameter definitions are no hard restriction, but my static analyzer (PHPStan) reports this as an issue:

task('app:my-task', fn () => run('whoami'));

because by default an arrow function returns from the function body.

Changing the parameter definitions of callables like this, would solve that problem:

// ...
 * @param callable():mixed|array|null $body Callable task, array of other tasks names or nothing to get a defined tasks
// ...

If you'd accept, I'll make a PR.