`gemini.web.js` : fetch vega-lite@4 , not v5/latest (effectively)

See comment: https://github.com/uwdata/gemini/issues/27#issuecomment-2209516407

I don't *immediately know* how to fix this other than [what I rushed into my fork](https://github.com/declann/gemini/commit/1c2e6790c479815759879716e0899d23fbe0a8fc)