Clear all cells / Run all cells

We should add options to run all cells and clear all cell outputs.
I don't think these need to be UI elements.
I think we should lean into doing this with code.
i.e. We should add suitable libraries/functions that can be run in the app console.

```
runme.clearOutputs(runme.getCurrentNotebook())
runme.runAll(runme.getCurrentNotebook())
```


