Stop using node-pty in Electron renderer process

[starpit]

This causes some issues:

1. Electron has limited support for non-context-aware native modules. We have had to set app.allowRendererProcessReuse =true, which results in

(electron) 'allowRendererProcessReuse' is deprecated and will be removed.

2. node-pty still does implement a context-aware module: non-context-aware native module microsoft/node-pty#405

3. Electron post v10 will no longer allow the allowRendererProcessReuse hack, thus pinning us to Electron v10

I think we can address this by running the pty in the main process. The plugin-bash-like/pty/{client,server}.ts architecture already supports this separation. We need only write a channel that connects over electron ipc.