Do not show welcome message if installed by Settings Sync feature of VS Code

[sandy081]

Hi, I am developer of VS Code and I am currently going through those extensions which has custom welcome experience (notifications / pages) on install. VS Code provides Settings Sync feature OOB and users can have their machines in sync using this feature. So, when user sync such extensions with custom welcome experience, they should not be seeing welcome experience again in their other machines. To enable this we recently added an API - microsoft/vscode#95209 (comment) for extensions to sync their global state when Settings Sync is enabled. It seems this extension is having custom welcome experience and not yet syncing their global state so that users won't see welcome notifications/pages when syncing extensions.

Hence filing this issue for this extension to adopt above API and make sure welcome experience is not repeated to users on syncing.

Here are the steps you can follow to make sure your extension is not showing welcome experience on sync:

• Machine 1 - Turn on Settings Sync feature in VS Code
• Machine 1 - Install alefragnani.bookmarks - Shows welcome (What's New in Bookmarks) page ✅

• Machine 2 - Turn on Settings Sync - this installs alefragnani.bookmarks extension
  🐛 Shows welcome page again

[sandy081]

I suppose this PR also fixes this - alefragnani/vscode-whats-new#24.