Add AST post-processing (desugaring) stage

[MaxGraey]

This extra stage run immediately after AST creation and process following tasks:

• Desugaring for top-level non-exported to host, non-mutable (const) FunctionExpression to FunctionDeclaration with preserving order.
• Destructive assignment desugaring
• Spread syntax desugaring
• Variadic function desugaring
• String enum desugaring (?)
• Processing for custom (user-space) decorators

[MaxGraey]

Another good candidate for simple rewriting is string operations:

const a = "a".charCodeAt(0);

to

const a = 0x61;

and

str.charAt(i) == 'e'

to

str.charCodeAt(i) == 0x65

rewrite:

= new Array
= new Array(?<T>)();
= new Array(?<T>)(0);

to

: T[] = [];
: T[] = [];
: T[] = [];

[MaxGraey]

Interesting paper about AST rule rewriting with partial parsing:
TWEAST: A Simple and Effective Technique to Implement Concrete-Syntax AST Rewriting Using Partial Parsing.pdf