ensure_ascii=False as default in to_json() and expose this option

[ckhung]

Enhancement Description

I want to use to_json() to study vega. There are zh_TW characters in my chart, and they are encoded into ascii. I suppose utf8 is quite standard now everywhere. It would help international users if to_json() sets ensure_ascii=False as default when calling json.dumps(), and then make this option exposed for users to turn it on if they need to avoid utf8 characters and create pure ascii output.

Checklist

• [v ] I checked for duplicate issues and found none.