Compiler API: Expose More from typescript.d.ts

**TypeScript Version:** 2.9.0-dev.20180426 While [Traversing the TypeScript AST](https://github.com/Microsoft/TypeScript/wiki/Using-the-Compiler-API#traversing-the-ast-with-a-little-linter) of a source file, there have been a number of helpful things I've found by extending `ts.*` from `typescript.d.ts` and I would like it if these could be exposed (or not marked `/* @internal */`). **Search Terms:** typescript.d.ts Node.locals Symbol.parent getContainingFunction isAssignmentTarget **Code** ```ts declare namespace ts { interface Node { /* @internal */ readonly locals?: SymbolTable; // Locals associated with node (initialized by binding) } interface Symbol { /* @internal */ readonly parent?: Symbol; // Parent symbol } function getContainingFunction(node: Node): SignatureDeclaration | undefined; function isAssignmentTarget(node: Node): boolean; } ``` Note: * Added `readonly` for `Node#locals` & `Symbol#parent` * For `Symbol#parent`, the goal is: given the symbol for a method (e.g., `methodSymbol` for `Number#toString`) find where that method is defined (e.g., `Number`)... maybe `program.getTypeChecker().getSymbolAtLocation(methodSymbol.valueDeclaration.parent)` is what is "supposed" to be used instead? * For `ts.getContainingFunction()` it seems more appropriate to have a return type of `SignatureDeclaration | undefined` instead of `SignatureDeclaration` **Related Issues:** #15841