feat: add property to show fill color for geojson

[jessicamcinchak]

Closes #63

When geojsonFill is true/present, it assumes your fill color is your stroke color with 20% opacity. This is pretty consistent with drawing style defaults! It's false by default, so geojson data will still display with only a border color by default.

Example:

<my-map geojsonColor="#286e8a" geojsonFill />
<script>
  const map = document.querySelector("my-map");
  map.geojsonData = {
    "type": "FeatureCollection",
    "features": [
      {
        "type": "Feature",
        "properties": {},
        "geometry": {
          "type": "Polygon",
          "coordinates": [
            [
              [
                -0.11562466621398924,
                51.50746034612789
              ],
              [
                -0.11449813842773436,
                51.50617151942102
              ],
              [
                -0.11359691619873047,
                51.50653212745768
              ],
              [
                -0.11348962783813477,
                51.50645199258505
              ],
              [
                -0.1127493381500244,
                51.50665900738443
              ],
              [
                -0.11383295059204103,
                51.507907754128546
              ],
              [
                -0.11562466621398924,
                51.50746034612789
              ]
            ]
          ]
        }
      }
    ]
  };

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[jessicamcinchak]

Update property boundary colour to be more prominent

[gunar]

Looks great!