Show a symbol in the gutter if there is a diagnostic on the line

[sonnyp]

Like Builder:

This is particularly helpful in Workbench since the editor width is limited (having multiple panes) so an error can easily happen outside the viewport (in the scroll view).

This will help beginners find errors in their code.

This should be reasonably easy to do using by placing a GtkSourceMark in the GtkSourceGutter. See https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/class.Gutter.html