Implement coordinate signing/normalization mechanism from #46

[lukas-h]

As described in comment: https://github.com/dartclub/turf_dart/issues/46#issuecomment-1057465724

The decision was, to keep Position unsigned by default, but it can be converted to a signed coordinate easily!

The imagined behaviour with signed/unsigned is similar to DateTime, which can be local or UTC. The DateTime has conversion functions toLocal() and toUTC(), that return a new object. Internally it uses a boolean to save the state (local = true/false).

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Tasks:

• [x] Implementation
• [ ] Documentation
• [ ] Tests

[lukas-h]

@armantorkzaban I will continue to work on this in the next days