Add Integer.Zero, Double.Zero, and String.Empty shared properties

Add read-only shared properties Integer.Zero, Double.Zero, and String.Empty for consistent, explicitly typed default values.

Currency, Decimal, and Duration already expose Zero, but Integer and Double do not. Code that builds an Array(Of Object) containing defaults for different types currently mixes named defaults with literals. These properties would make that code more consistent and make the intended numeric type explicit.

Expected values:

• Integer.Zero returns the Integer value 0.
• Double.Zero returns the Double value positive 0.0.
• String.Empty returns the empty String "".

Existing literals and default initialisation remain supported and unchanged.