`hasHandlers` looks through all parent loggers,
whereas we only want to check handlers specifically on the logger

This commit restructures the execution phase,
to act as a notebook client context manager,
within which the renderer runs.
This will allow for a kernel client to be running during rendering,
and thus for it to be queried for variables.

Make different execution methods subclasses of `NotebookClientBase`

This allows it to work with "inline" execution