Refactor jats-importer to smaller chunks

Divides the jats-importer.ts into smaller, more maintainable pieces that try to utilize pure functions instead of mutating variables. Also fixes bug with duplicate IDs when assigning new ones (due to referencing issue caused by node.attrs.id = nextID). Feature and performance-wise the functionality should be identical although the improved parsing causes the iteration over the doc take even longer (encoding step being probably the main culprit).