Implement language features in cells of polyglot notebook

[gjsjohnmurray]

When using the Polyglot IRIS kernel, all code cells use the iris-polyglot language (display name 'Polyglot IRIS notebook'). AFAIK the .ipynb notebook format and the Jupyter ecosystem doesn't yet support setting code language per cell.

Server-side, our polyglot kernel looks for a %%XXX cell-magic line at the start of a cell when deciding how to execute its contents.

By using the request forwarding technique we could implement language features in the VS Code notebook, notably code colouring.