Readability & size improvements for JS AssetLibrary

One line per asset instead of previous 3.
50-70% fewer bytes per asset declaration.

Before (187 bytes):
````javascript
DefaultAssetLibrary.path.set("img/sample.png","img/sample.png");
var value = Reflect.field(openfl.AssetType,"image".toUpperCase());
DefaultAssetLibrary.type.set("img/sample.png",value);
````
After (60 bytes):
````javascript
this.addExternal("img/sample.png","image","img/sample.png");
````