Use progress notification for "Installing ..." message

[jrieken]

Describe the bug

• first run experience
• download and run extension for the first time
• you see notification message about installing something

Instead, use the withProgress-API, with the Notfication-location. That will ensure a better UX in this case

[jrieken]

also see our UX guidelines: https://code.visualstudio.com/api/references/extension-guidelines#progress-notification