Derive icon paths (#17816)

This PR improves adding and working with icons by using the new
`DerivePathStr` to derive icon paths.

This means paths no longer need to be manually specified, and the
`IconName` and file name will always be consistent between icons.

This PR does not do any work to standardize icons visually, remove
unused icons, or any other such cleanup.

Release Notes:

- N/A

crates/diagnostics/src/items.rs

@@ -30,7 +30,7 @@ impl Render for DiagnosticIndicator {
             (0, warning_count) => h_flex()
                 .gap_1()
                 .child(
-                    Icon::new(IconName::ExclamationTriangle)
+                    Icon::new(IconName::Warning)
                         .size(IconSize::Small)
                         .color(Color::Warning),
                 )
@@ -52,7 +52,7 @@ impl Render for DiagnosticIndicator {
                 )
                 .child(Label::new(error_count.to_string()).size(LabelSize::Small))
                 .child(
-                    Icon::new(IconName::ExclamationTriangle)
+                    Icon::new(IconName::Warning)
                         .size(IconSize::Small)
                         .color(Color::Warning),
                 )