No access to local files in interactive vega blocks

[kachkaev]

There seems to be some access issue in the VSCode preview. When an interactive vega block refers to a local file, the request fails. To reproduce:

1. Create data.csv with the following contents:

   x,y
   10,55
   20,35
   30,80
   

2. Create problem.md with the following contents:

```vega-lite
{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": { "url": "data.csv" },
  "encoding": {
    "x": { "field": "x", "type": "quantitative" },
    "y": { "field": "y", "type": "quantitative" }
  },
  "mark": "circle"
}
```

```vega-lite interactive
{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": { "url": "data.csv" },
  "encoding": {
    "x": { "field": "x", "type": "quantitative" },
    "y": { "field": "y", "type": "quantitative" }
  },
  "mark": "circle"
}
```

Result

Expected result

↑ the same example works correctly in Atom

I’m wondering if there is any flag we can tweak while using VSCode API to allow access to local files 🤔

[shd101wyy]

It might be related to the file protocol change but I am not exactly sure. They seem to change to use vscode-webview-resource:// or something...

[jwoLondon]

Do you think this looks like a solvable problem? Currently this is causing a significant block at our end.