Use new function type syntax

It would be great if we could support the [new function type](https://github.com/HaxeFoundation/haxe-evolution/blob/master/proposals/0003-new-function-type.md) since this would provide richer completion/documentation for the working with JavaScript. I noticed this information is already available in the webidls.

For example; [BaseAudioContext webidl](https://github.com/HaxeFoundation/html-externs/blob/52a15145a2b036c389b422a7d694a90aaa7a66dd/webidl/mozilla/BaseAudioContext.webidl#L43-L45):
```idl
callback DecodeSuccessCallback = void (AudioBuffer decodedData);
callback DecodeErrorCallback = void (DOMException error);

...

Promise decodeAudioData(ArrayBuffer audioData,
optional DecodeSuccessCallback successCallback,
optional DecodeErrorCallback errorCallback);
```
Currently we generate:
```haxe
function decodeAudioData(audioData:js.lib.ArrayBuffer, ?successCallback:AudioBuffer->Void, ?errorCallback:js.html.DOMException->Void):Promise;
```
This could be improved like this:
```haxe
function decodeAudioData(audioData:js.lib.ArrayBuffer, ?successCallback:(decodedData:AudioBuffer) -> Void,
?errorCallback:(error:DOMException) -> Void):Promise;
```
.. or with typedef
```haxe
function decodeAudioData(audioData:js.lib.ArrayBuffer, ?successCallback:DecodeSuccessCallback, ?errorCallback:DecodeErrorCallback):Promise;

typedef DecodeSuccessCallback = (decodedData:AudioBuffer) -> Void;
typedef DecodeErrorCallback = (error:DOMException) -> Void;
```

If the info is not available we can always fallback to the "old syntax".