Widgets for TypeAugmentation, member methods and patterns

[kaashyapan]

#43

[edgarfgp]

@kaashyapan I Finally had some time to go over this PR. Thanks for taking the time. I made some changes that I will describe below:

• The aim of Fabulous. AST is not to add a DSL for all the Oak Nodes, But rather add the ones that make sense based on
  the users and how it can be done using the Fabulous internals
• I think it makes send to add Widgets for MemberDefn as these can be used in Classes, Records, Unions etc.

[<RequireQualifiedAccess; NoEquality; NoComparison>]
type MemberDefn =
    | ImplicitInherit of InheritConstructor
    | Inherit of MemberDefnInheritNode
    | ValField of FieldNode
    | Member of BindingNode
    | ExternBinding of ExternBindingNode
    | DoExpr of ExprSingleNode
    | LetBinding of BindingListNode
    | ExplicitCtor of MemberDefnExplicitCtorNode
    | Interface of MemberDefnInterfaceNode
    | AutoProperty of MemberDefnAutoPropertyNode
    | AbstractSlot of MemberDefnAbstractSlotNode
    | PropertyGetSet of MemberDefnPropertyGetSetNode
    | SigMember of MemberDefnSigMemberNode

• I do not think adding Widgets for Pattern is not a good idea for now. We could just add extensions to make it easy to create those. Same for Expr

Regarding naming convention:

• For Widget constructors, I think I use a more natural language i.e.
  • TypeDefnRegularNode -> Class, GenericClass
  • TypeDefnRegularNode -> Interface, GenericInterface
• For widgets modifiers, I think it makes sense to match the Oak AST model