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.

src/sql/workbench/parts/notebook/cellViews/stdin.css

@@ -0,0 +1,18 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+stdin-component {
+	display: flex;
+	flex-flow: row;
+	padding: 10px;
+	align-items: center;
+}
+
+stdin-component .prompt {
+	flex: 0 0 auto;
+}
+stdin-component .input {
+	flex: 1 1 auto;
+	padding-left: 10px;
+}