turf-tag: Allow specifying feature level attributes as fields

turf-tag currently takes a field and outField which it expects to be an an attribute of the properties object.

In some cases however, the desired field may be a feature level attribute not part of the property object like id in this example:

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "id": 12345,
      "properties": {},
      "geometry": {
       ...
      }
    }
  ]
}

Its currently not possible to target such attributes.

A way to specify the field refers to a feature level attribute will be very useful, especially while working with Mapbox vector tiles which commonly has feature level ids.

Start with the turf-tag implementation and documentation to understand how field and outField currently read from properties. Define how callers identify feature-level attributes such as id, then verify that property-based fields continue to work and that the documented example is supported.