rough & minimal changes to get index.ts to compile

[parmentelat]

with these changes I can successfully compile index.ts and thus move to pip install -e .

I am no jlab extension expert myself, I just happen to have written my first one a few weeks ago (on jlab4 too), and it had to do with metadata a lot, so I thought I could help out a bit with that

note that this is the only scope for this PR:

• the code dealing with metadata should maybe work as is
• and as far as the very last change (in the factory definition) this is really just a wild guess in order to get the compilation to pass, but it's probably not the right thing to do

hope this helps

PS. see also e.g.
https://www.jsdocs.io/package/@jupyterlab/notebook#NotebookModel.getMetadata