Notebook StdIn support to fix #5231 (#5232)

Fixes #5231 
- Add stdin handling. Has to be at UI level so add plumb through handling
- Add unit tests
- Add new StdIn component.

Testing:
Unit Tests and manual testing of following:
- Prompt for password using `getpass` in python.
   - Password prompt is hidden since "password" is true.
   - Hit enter, it completes
- prompt, stop cell running, StdIn disappears
- prompt, hit escape, stdIn disappears and stdIn request is handled.

Issues: focus isn't always set to the input even though we call focus.
Will investigate this further.

extensions/notebook/src/jupyter/jupyterKernel.ts

@@ -21,7 +21,7 @@ function toStdInMessage(msgImpl: KernelMessage.IStdinMessage): nb.IStdinMessage
 	return {
 		channel: msgImpl.channel,
 		type: msgImpl.channel,
-		content: msgImpl.content,
+		content: <any>msgImpl.content,
 		header: msgImpl.header,
 		parent_header: msgImpl.parent_header,
 		metadata: msgImpl.metadata