5.3 Let and Const Declarations

Let and const declarations are extended to include optional type annotations.

  LexicalBinding: ( Modified )   SimpleLexicalBinding   DestructuringLexicalBinding

  SimpleLexicalBinding:   BindingIdentifier TypeAnnotation_opt Initializer_opt

  DestructuringLexicalBinding:   BindingPattern TypeAnnotation_opt Initializer_opt

TODO: Document scoping and types of let and const declarations.