[AstAnalyser] Add initialize and finalize callback to manipulate SourceFile

[fraxken]

Current AstAnalyser as flaw when we want to manipulate the SourceFile class before or after walking the AST, here is an example where we add new custom tracing;

import { AstAnalyser } from "@nodesecure/js-x-ray";

const scanner = new AstAnalyser();

scanner.analyse("const foo = 'bar';", {
 initialize(sourceFile) {
   sourceFile.tracer.trace("...");
 },
 finalize(sourceFile) {
   // do something
 }
});

Those callbacks should be triggered before and after walking the AST