Replace string manipulations (`split(..).pop()`, etc) with real URL helpers

[julrich]

Instead of doing stuff like:

const schemaPointer = subSchema.$ref.split('#').pop();
const schemaId = subSchema.$ref.split('#').shift();

we could do stuff like this, in more places:

const targetSchemaURL = new URL(targetSchemaId);
const layeringSchemaURL = new URL(schemaId);

const targetSchemaURLPathParts = targetSchemaURL.pathname.split('/');
const layeringSchemaURLPathParts = layeringSchemaURL.pathname.split('/');