Use `PrecisionModel` for serializing/deserializing coordinates (STJ)

[MizardX]

• Serialization and deserialization of the Envelope type moved to it's own converter, so that it easily can use a PrecisionModel parameter.
• Extension methods for Utf8JsonWriter and Utf8JsonReader that uses a PrecisionModel parameter.
• Serialization and deserialization of coordinates uses the extension methods with the current PrecisionModel.
• Fixes #135

I am not sure if PrecisionModel should be used for the Z coordinate. This is something I want, but this is not supported by the Newtonsoft GeoJSON serializer.

[CLAassistant]

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