UI: "whats new" / "welcome" banner in Explorer, Q panels

## Problem

Currently when the Toolkit is updated it shows a "Toolkit updated..." popup: https://github.com/aws/aws-toolkit-vscode/blob/489b4e26032a588ccd4a53b98a78b6b08ca3da2b/src/shared/extensionUtilities.ts#L344

- This is a bit intrusive.
- The "View quickstart" button is easily missed.
- The quickstart isn't effective at showcasing new features

## Proposal

1. Do not show _any_ notification until the "AWS" icon is clicked.
1. Instead of a popup, use the [viewsWelcome](https://code.visualstudio.com/api/references/contribution-points#contributes.viewsWelcome) to show:
- highlight 2-3 key features of the updated Toolkit
- link to full changelog

Start at src/shared/extensionUtilities.ts around the existing “Toolkit updated...” notification, then review the VS Code viewsWelcome contribution-point documentation and the Explorer and Q panel views. Done means no update notification appears before the AWS icon is clicked, and the relevant welcome view highlights 2–3 updated features and links to the full changelog.

Written by the indexing model from the issue text.