Code execution cancellation

[alex-ketch]

This PR adds logic for emitting code execution and cancellation events.

[nokome]

@alex-ketch Can you please,

• [x] Rename ExecutableDocumentToolbar to DocumentToolbar throughout
• [x] Consider having the run button in the toolbar have the option to run in appearance or topological order (as an escape hatch in case topo order gets broken)
• [x] Add a "Cancel" button to the toolbar which will fire a stencila-execute-cancel event with nodeId == null and scope == All
• [x] Add a "Restart" button to the toolbar which will fire a stencila-kernel-space-restart (or similarly named) event
• [x] Over in the stencila/stencila repo hook up the cancel and restart events with documents.cancel and documents.restart and remove the temporary functions that I added for testing:

  // Temporary functions for testing in the console
  // @ts-ignore
  window.stencilaExecute = async (
    nodeId: null | string,
    ordering: 'Single' | 'Appearance' | 'Topological'
  ) => {
    const [client, document] = await startup()
    await documents.execute(client, document.id, nodeId, ordering)
  }
  // @ts-ignore
  window.stencilaCancel = async (
    nodeId: null | string,
    scope: 'Single' | 'All'
  ) => {
    const [client, document] = await startup()
    await documents.cancel(client, document.id, nodeId, scope)
  }
  // @ts-ignore
  window.stencilaRestart = async () => {
    const [client, document] = await startup()
    await documents.restart(client, document.id)
  }

• [ ] Think about how we might expose the ordering option for execute and the scope option for cancel to the UX for individual CodeChunk and CodeExpression nodes. In the short term having a key modifier e.g. Click == cancel, Shift+Click == cancel all might be better than nothing and exposes the alternatives. If we don't make progress on this at least having it parked in an issue in this repo would be useful.