Extend hints and autocompletion

Autocompletion has already been enabled for variables, see #84 (closed) and !67 (merged).

IDEs provide more powerful hints and autocompletion functions. We should investigate how far this can be done in Jupyter.

For example, a very helpful hint is to show a list of the next valid rules.

Again, we we will have to look at existing kernels to see how this is implemented there.

• https://ipython.readthedocs.io/en/stable/development/wrapperkernels.html#optional-steps
• https://github.com/jupyter/jupyter/wiki/Jupyter-kernels
• https://jupyter-client.readthedocs.io/en/latest/kernels.html