A flag to resolve codeSelect using ASTParser

Introduce a CompilationUnit.DOM_BASED_OPERATIONS flag which instead of
using the ECJ-derived parser uses ASTParser to create a DOM and then
uses the DOM to implement codeSelect

This has the benefit of:
* Simplifying the code as an crawling DOM is usually simpler than
dealing with lower-level parser
* allowing other parsers as backend if ASTParser is configured for it