Make some dependencies optional

[jtpio]

In the main plugin entrypoint, a couple of dependencies are marked as requires:

https://github.com/QuantStack/jupyterlab-drawio/blob/938cc9271d4ae1bca2599368a280150f404da9fb/src/index.ts#L68

Ideally ILayoutRestorer, ILayoutRestorer and ICommandPalette could be set as optional instead, which would make using jupyterlab-drawio in other lab distributions easier.