Enable automatic diagram preview

[LuboTerifaj]

What would you like to be added:

Enable automatic diagram preview.

The diagram should automatically open when you open the workflow file.

Why is this needed:

If you open any serverless workflow file (*.sw.json, *.sw.yml), you have to open the Command Palette (⇧⌘P or Ctrl + Shift + P), then type and select the option "Serverless Workflow: Preview Diagram" to see the workflow's diagram.

[JBBianchi]

Or just right-click on the file in the explorer or inside the editor.

[JBBianchi]

@LuboTerifaj would you be so kind to test #30 please ?