Add method to check if variable is of type array

Currently, there is no way to check whether a variable is an array in JS resolver functions.

Here are a couple of approaches as to how you would go about it in a browser or Nodejs environment:

1. Use `Array.isArray`, which doesn't exist in `APPSYNC_JS`.
2. Use `arr.constructor === Array`. That doesn't work, because `arr.constructor` is undefined in `APPSYNC_JS`.
3. Check for multiple array functions. Ex:
```js
function isArray(x) {
return (
typeof x.find === 'function' &&
typeof x.length === 'number' &&
typeof x.sort === 'function'
);
}
```
This also doesn't work because `typeof` any function is `'undefined'` in `APPSYNC_JS`. This is also (IMO) the worst solution.

The functionality is present in `vtl` resolvers with `$util.isList`, so it would be nice if it was available in the JS runtime as well.

No files or tests are named in the issue. Start by locating the APPSYNC_JS resolver runtime and compare its available type-checking helpers with VTL’s `$util.isList`; done should mean JS resolver functions can reliably determine whether a value is an array.

Written by the indexing model from the issue text.