✨ Add IEquatableQuantity

Ref #1193

In v5, equality changed to strict equality.
This meant it became more cumbersome to compare equality for `IQuantity` objects of different units, but similar value when converted to the same unit.

- Add interface `IEquatableQuantity` to help compare `IQuantity` objects with a tolerance for error

Author: angularsen

CodeGen/Generators/UnitsNetGen/QuantityGenerator.cs

@@ -75,6 +75,7 @@ namespace UnitsNet
         IComparable<{_quantity.Name}>,
         IConvertible,
         IEquatable<{_quantity.Name}>,
+        IEquatableQuantity<{_quantity.Name}>,
         IFormattable");
 
             Writer.WL($@"