added "Vega Preview" a Visual Studio Code extension to applications.md (#3894)
added "Vega Preview" a Visual Studio Code extension to applications.md (#3894)



uwdata/vega-lite,uwdata/vega-lite,vega/vega-lite,vega/vega-lite,vega/vega-lite,uwdata/vega-lite,uwdata/vega-lite,uwdata/vega-lite,vega/vega-lite,vega/vega-lite