Improve notebook colors and UX (#3914)

This was reviewed / worked on with Smitha and will be signed off on by PM via mail.
1 thing left (make run button look better when not selected) will be one in separate review.

Changes
- Add top/bottom padding to editor so it's not cramped
- Added an (on by default) setting `notebook.overrideEditorTheming`. This controls whether new colors etc. are used for notebook editors or if users should see vanilla UI like in standard editor. Settings under this flag are:
  - When unselected, editor has same color as toolbar. On selection it goes back to regular editor view so colors work "right"
  - In standard light/dark themes we now use a filled in background color instead of border box.

extensions/notebook/package.json

@@ -26,6 +26,11 @@
 					"type": "boolean",
 					"default": false,
 					"description": "%notebook.sqlKernelEnabled.description%"
+				},
+				"notebook.overrideEditorTheming": {
+					"type": "boolean",
+					"default": true,
+					"description": "%notebook.overrideEditorTheming.description%"
 				}
 			}
 		},