Adds the ES6 transformer

[rbuckton]

The ES6 transformer is designed to transform most ES6-specific syntax into compatible down-level JavaScript, including:

• Class Declarations
• Class Expressions
• Arrow Functions
• Parameter Initializers
• Binding Patterns
• Rest Parameters
• Destructuring Assignments
• Spread for Arrays, Call Expressions, and New Expressions
• Let/Const declarations
• For..Of loops
• Shorthand Property Assignments in Object Literals
• Methods in Object Literals
• Templates

Related Pull Requests:

• Adds the visitor API foundation for tree transformations. #6892 - Adds the visitor API foundation for tree transformations.
• Adds the Transform Flags concept for tree transformations #6983 - Adds the Transform Flags concept for tree transformations.
• Adds the transformFiles API for tree transformations #6986 - Adds the transformFiles API for tree transformations.
• Adds a simplified pretty printer for tree transformations #6987 - Adds a simplified pretty printer for tree transformations.
• Adds the TypeScript transformer #7191 - Adds the TypeScript transformer.
• Adds the JSX transformer #7192 - Adds the JSX transformer.
• Adds the ES7 transformer #7193 - Adds the ES7 transformer.
• Adds the ES6 transformer #7194 - Adds the ES6 transformer.
• Adds the Module transformers #7206 - Adds the Module transformer.