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/collab_ui/src/collab_panel/channel_modal.rs

@@ -168,7 +168,7 @@ impl Render for ChannelModal {
                             .w_px()
                             .flex_1()
                             .gap_1()
-                            .child(IconElement::new(Icon::Hash).size(IconSize::Medium))
+                            .child(Icon::new(IconName::Hash).size(IconSize::Medium))
                             .child(Label::new(channel_name)),
                     )
                     .child(
@@ -406,7 +406,7 @@ impl PickerDelegate for ChannelModalDelegate {
                                 Some(ChannelRole::Guest) => Some(Label::new("Guest")),
                                 _ => None,
                             })
-                            .child(IconButton::new("ellipsis", Icon::Ellipsis))
+                            .child(IconButton::new("ellipsis", IconName::Ellipsis))
                             .children(
                                 if let (Some((menu, _)), true) = (&self.context_menu, selected) {
                                     Some(