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/feedback/src/deploy_feedback_button.rs

@@ -1,5 +1,5 @@
 use gpui::{Render, ViewContext, WeakView};
-use ui::{prelude::*, ButtonCommon, Icon, IconButton, Tooltip};
+use ui::{prelude::*, ButtonCommon, IconButton, IconName, Tooltip};
 use workspace::{item::ItemHandle, StatusItemView, Workspace};
 
 use crate::{feedback_modal::FeedbackModal, GiveFeedback};
@@ -27,7 +27,7 @@ impl Render for DeployFeedbackButton {
                 })
             })
             .is_some();
-        IconButton::new("give-feedback", Icon::Envelope)
+        IconButton::new("give-feedback", IconName::Envelope)
             .style(ui::ButtonStyle::Subtle)
             .icon_size(IconSize::Small)
             .selected(is_open)