Provide a longturtle save format

Currently, saving to a file uses the n3 turtle serialiser, which prioritises speed over readability. It's also not deterministic and makes working with git diffs difficult.

Instead, provide options to use different serialisers when saving.

- turtle (fast) - uses the n3 serialiser
- longturtle - slow, uses pyscript/pyodide, but provides deterministic and pretty formatting