Get rid of reflection for parsing

The [ScalarBuilder](https://github.com/atmoos/Quantities/blob/main/source/Atmoos.Quantities/Core/Construction/ScalarBuilder.cs) relies heavily on reflection. This is really ugly and ought to be improved.

Consider letting units implement an interface that looks something like this:

```csharp
public interface IUnitFactory where TSelf : IUnit, IDimension
{
static abstract Creator GetCreator(/* ISystems ?? */);
}
```

Relates to #72

Start with source/Atmoos.Quantities/Core/Construction/ScalarBuilder.cs to map where reflection is used during parsing, then review the related discussion in issue #72. Compare that flow with the proposed IUnitFactory interface and determine the required creator and systems contracts. Done means parsing no longer depends on the current reflection-heavy approach while preserving the library's typesafe quantity behavior.

Written by the indexing model from the issue text.