[tfjs-vis] investigate vega-lite-api

[tafsiri]

Investigate https://github.com/vega/vega-lite-api this may give us beter type checking and increase the readability maintainability of our vega-lite specs.

[gaikwadrahul8]

Hi, @tafsiri

Apologize for the delayed response and I see we have updated our official documentation w.r.t https://js.tensorflow.org/api_vis/1.5.1/ so may be we have taken care of this issue if I'm not wrong. Thank you!

CC :@pyu10055