153. Promisify

Implement a utility similar to util.promisify for Node-style callbacks.

Implement:

function promisify(fn, thisArg) {}

Behavior:

• Return a new function that wraps fn in a Promise
• Assume callback style is (err, value)
• If err is truthy, reject
• Otherwise resolve with value
• Forward all input arguments
• If thisArg is provided, call fn with that context