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/feedback_modal.rs

@@ -488,7 +488,7 @@ impl Render for FeedbackModal {
                     .child(
                         Button::new("community_repository", "Community Repository")
                             .style(ButtonStyle::Transparent)
-                            .icon(Icon::ExternalLink)
+                            .icon(IconName::ExternalLink)
                             .icon_position(IconPosition::End)
                             .icon_size(IconSize::Small)
                             .on_click(open_community_repo),