Make some dependencies optional

[jtpio]

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

jupyterlab-drawio/src/index.ts

Line 68 in 938cc92

requires: [IFileBrowserFactory, ILayoutRestorer, IMainMenu, ICommandPalette],

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