add options or settings to change vega libraries versions

[RandomFractals]

for testing older vega specs with this extension

[RandomFractals]

from our prev. chats with @domoritz & vega devs in their slack & ideas I floated on this in the past:

read vega json schema v. and load the corresponding vega libs in VV webview ...

will not be available offline or in disconnected mode the way you can use VV now with built-in vega libs and examples, but it sounds like some devs using legacy vega(s) json specs are interested in this feature for testing their down-stream vega apps, maps and graphs :)

[RandomFractals]

Closing this feature request as VSCode marketplace lists older versions of this extension and I'll do my best to update vega libraries and built-in examples for major new releases.