[whatwg/streams] Pass controller to transformer methods in TransformStream (#518)

Currently methods of the transformer object are passed individual parameters for each action they can take. For example, the transform method is called like this:

```javascript
transform(chunk, done, enqueue, closeReadable, error)
```

where `done`, `enqueue`, `closeReadable` and `error` are all functions that `transform()` can call in processing `chunk`.

For consistency with ReadableStream and WritableStream, it would be better if the methods were passed a controller object. The definition would be something like

```javascript
class TransformStreamDefaultController {
  constructor(stream)

  done()
  enqueue(chunk)
  closeReadable()
  error()
}
```

In a Promise-based implementation `done()` and `error()` would go away.

Then the signature of `transform()` would change to

```javascript
transform(chunk, controller)
```

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