Fix disabled Save button on VSCode "File > Save" menu

On the latest VSCode versions, the File > Save menu is disabled for Webviews (the kind of 'tab' we run DMN/BPMN editors on). There's a proposed API to open Webviews as actual editors and from initial tests this will re-enable the File > Save menu. We should assess the possibility of using this API to make sure that the option to save using the mouse comes back. Using keyboard shortcuts to save still works.