JupyterLite support

[jtpio]

Since JupyterLite does not support server extensions, maybe there could be an option to do the indexing of files via calls to serviceManager.contents.get().

In JupyterLite these don't end up being real HTTP calls but function calls instead, so it could be a viable option for now.

The regular would still default to using the Jupyter Server extension, although it could use the serviceManager.contents.get() approach too if enabled.