add gistpad.viewers contribs to package.json

[RandomFractals]

for gist pad to default to geo data viewer for keplergl maps and configs display from those gists.

per @LostInTangent directiives, this is what needs to be added:

"contributes": {
   "gistpad.viewers": [
      {
        "filenames": [
          "keplergl.json"
        ],
        "command": "map.view"
      }
    ]
}

[RandomFractals]

@LostInTangent this works for explicit filenames only ...

making it work for the rest would be nice!

"gistpad.viewers": [
      {
        "filenames": [
          ".geo.json",
          ".geojson",
          ".kgl.json",
          "keplergl.json",
          "keplergl_time.json",
          ".map.json"
        ],
        "command": "map.view"
      }
    ]

I think you should handle it on your end with better file names & ext's pattern matching approach ...