repl: Add possibility to stop a cell while it's running

[raideno]

Body

What are you proposing?

Currently when working with python notebooks, if we run a cell, we can stop it while its running using cmd-c or by clicking the button on the bottom right which isn't very visible.

It would be nice to have the run button be transformed into a stop button while the cell is running and be able to stop the cell directly using that button.

Why does this matter?

Making ZED notebook editor more consistent with other editors.

Are there any examples or context?

The stop button should replace the "start" button on the left of the cell.