add csv, json & arrow data panel preview

[RandomFractals]

• map new data preview command to [vega.]data.preview & ctrl+alt+d command shortcut
• use this Observable js notebook & https://github.com/fin-hypergrid/core || better yet https://github.com/finos/perspective for that data grid display:

https://observablehq.com/@randomfractals/vega-datasets

[RandomFractals]

made more progress on this today: https://github.com/fin-hypergrid/core/issues/804#issuecomment-493700624

will package it for that v5.0 release soon!