Add simpler findReferences and getDefinitions that only returns nodes

[dsherret]

Right now findReferences and getDefinitions() is the same as in the compiler API and it's kind of a pain to use if you only want to get the nodes:

const classDeclaration = ...; // get a class or some other declaration somehow
const referencedSymbols = classDeclaration.getNameNode().findReferences();
const referencingNodes = referencedSymbols
    .map(s => s.getReferences())
    .reduce(/*...etc..*/)
    .filter(r => !r.isDefinition())
    .map(r => r.getNode()))

Would be nice if there was also a method for getting just the nodes (ex. classDeclaration.getReferencingNodes() and node.getDefinitionNodes()).

[dsherret]

• NamedNode and NameableNode - Added .getReferencingNodes() and findReferences().
• Identifier - Added getDefinitionReferencingNodes() and getDefinitionNodes()