Extract UI elements from storybook into new ui crate (#3008)

This PR extracts the various UI elements from the `storybook` crate into
a new `ui` library crate.

Release Notes:

- N/A

crates/ui/src/components/facepile.rs

@@ -0,0 +1,28 @@
+use gpui2::elements::div;
+use gpui2::style::StyleHelpers;
+use gpui2::{Element, IntoElement, ParentElement, ViewContext};
+
+use crate::{theme, Avatar};
+
+#[derive(Element)]
+pub struct Facepile {
+    players: Vec<Avatar>,
+}
+
+pub fn facepile(players: Vec<Avatar>) -> Facepile {
+    Facepile { players }
+}
+
+impl Facepile {
+    fn render<V: 'static>(&mut self, _: &mut V, cx: &mut ViewContext<V>) -> impl IntoElement<V> {
+        let theme = theme(cx);
+        let player_count = self.players.len();
+        let player_list = self.players.iter().enumerate().map(|(ix, player)| {
+            let isnt_last = ix < player_count - 1;
+            div()
+                .when(isnt_last, |div| div.neg_mr_1())
+                .child(player.clone())
+        });
+        div().p_1().flex().items_center().children(player_list)
+    }
+}