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/assistant/src/terminal_inline_assistant.rs

@@ -585,7 +585,7 @@ impl Render for PromptEditor {
                     .gap_2()
                     .child(ModelSelector::new(
                         self.fs.clone(),
-                        IconButton::new("context", IconName::SlidersAlt)
+                        IconButton::new("context", IconName::SettingsAlt)
                             .shape(IconButtonShape::Square)
                             .icon_size(IconSize::Small)
                             .icon_color(Color::Muted)