added configuration for diagnostic icons

This is to aid accessibility.

Some themes are not gread for colourblind people. With this
configuration, they can easily change the icons that appear around the
editor (gutter, statusbar) to make them easier to distinguish!

small fix

helix-view/src/gutter.rs

@@ -83,7 +83,7 @@ pub fn diagnostic<'doc>(
                     Some(Severity::Info) => info,
                     Some(Severity::Hint) => hint,
                 };
-                write!(out, "{}", indicator).ok();
+                out.push(indicator);
                 style
             })
         },