Fix Deprecated VSCode-API Calls

[fabianhauser]

Currently, multiple deprecation messages are shown:

Extension 'correctnessLab.dafny-vscode' uses a document selector without scheme. Learn more about this: https://go.microsoft.com/fwlink/?linkid=872305

Extension 'correctnessLab.dafny-vscode' uses the 'vscode.previewHtml' command which is deprecated and will be removed. Please update your extension to use the Webview API: https://go.microsoft.com/fwlink/?linkid=2039309

These calls should be fixed according to the Microsoft documentation.

[fabianhauser]

The previewHtml command will be disabled in the vscode march release

[fabianhauser]

To use the dotfile support in the futures, larger reimplementation efforts with the new WebView API would have been required. As not many users utilised this feature (and the graph can still be generated manually on the command line), we decided to remove it for now (see c777256).