Enhance IExtent type with additional params

[gowin20]

The IExtent interface is used to document map extents, as well as geometries with an esriGeometryEnvelope type. After working with this type, I believe a couple of minor enhancements can be made to help it align more closely to Esri's geometry specification.

1. Add the following optional properties

{
mmin?: number,
mmax?: number,
idmin?: number,
idmax?: number
}

These may be present on esriGeometryEnvelope objects, and are documented here: https://developers.arcgis.com/rest/services-reference/enterprise/geometry-objects/#envelope

2. Export the alias IEnvelope

• As the majority of geometries used for this type have a geometryType of esriGeometryEnvelope, another appropriate name for this interface is IEnvelope.