Add named constants (enums) to Small Basic

[McSherry]

This issue stems from a Yammer discussion.

At the moment, properties such as TextWindow.ForegroundColor take a string colour name. This is a poor experience for learners as IntelliSense cannot currently suggest correct values. This occurs at various points in Small Basic's API.

To fix this, Small Basic should support named constants (i.e. enum values) so that the compiler has a defined set of values it can suggest. This would be best implemented building over the existing enum infrastructure used by C# etc.

To improve user experience, it may be worth considering:

• Allowing the declaring type to be omitted (so e.g. ConsoleColor.Cyan could be written as Cyan alone in a context that accepts a named constant value)
• Warning if a named constant and variable name clash, but only if a named constant is used (so that programs that don't use the named constants don't have variable names clobbered)
• Including a new Small Basic object to turn strings into named constant values