Add markdown preview refresh command

[mjbvz]

Feature Request
Add a command to manually refresh the markdown preview

[mjbvz]

To verify:

1. In a workspace with a markdown file, create a style.css file that sets the background color:

body {
    background: hotpink !important;
}

1. Set "markdown.styles": [ "./style.css" ]
2. Open the markdown preview to the side
3. With the preview still open, edit the css file in the other panel to change the background color.
4. Verify that the refresh command updates the preview. Refresh should be both in the command palette and in the ... in the preview's status bar

[bpasero]

@mjbvz the refresh command in the "..." works, but the global command does not: