Unparse function

We should add a function that serializes an arbitrary object to SHON.

```go
func Unparse(obj any) ([]string, error)
```

The playground can become bidirectional between JSON and SHON.

Look at the existing parsing code to understand the SHON format. The new Unparse function must serialize any Go object to SHON strings. Start by examining the public API and existing tests to see how objects are currently parsed. Write tests for Unparse to verify it produces valid SHON from various inputs, then implement the function to match the format.

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