Add CSV toolbar #15

We need a toolbar to customize some file reading/parsing options.
So far this is mostly needed for CSV:

• selecting the delimiter
• We can add other options from read_csv

For example the JupyterLab built-in CSV viewer has such a toolbar

https://github.com/jupyterlab/jupyterlab/blob/main/packages/csvviewer/src/toolbar.ts

When using Arbalister, the CSV viewer can be be open with a right-click on the file