Rename IconElement to just Icon (#3974)

This PR renames the `IconElement` component to just `Icon`.

This better matches the rest of our components, as `IconElement` was the
only one using this naming convention.

The `Icon` enum has been renamed to `IconName` to free up the name.

I was trying to come up with a way that would allow rendering an
`Icon::Zed` directly (and thus make the `IconElement` a hidden part of
the API), but I couldn't come up with a way to do this cleanly.

Release Notes:

- N/A

crates/ui/src/components/disclosure.rs

@@ -1,6 +1,6 @@
 use gpui::ClickEvent;
 
-use crate::{prelude::*, Color, Icon, IconButton, IconSize};
+use crate::{prelude::*, Color, IconButton, IconName, IconSize};
 
 #[derive(IntoElement)]
 pub struct Disclosure {
@@ -32,8 +32,8 @@ impl RenderOnce for Disclosure {
         IconButton::new(
             self.id,
             match self.is_open {
-                true => Icon::ChevronDown,
-                false => Icon::ChevronRight,
+                true => IconName::ChevronDown,
+                false => IconName::ChevronRight,
             },
         )
         .icon_color(Color::Muted)