Add stories for collab notifications (#3967)

This PR adds some basic stories for collab notifications to make them
easier to work on:

<img width="1076" alt="Screenshot 2024-01-08 at 9 43 39 PM"
src="https://github.com/zed-industries/zed/assets/1486634/4a0adcfa-1134-49c2-b589-74ac1d52af4c">

I factored out a `CollabNotification` component that defines the general
structure for one of these notifications, and this is the component that
we use in the stories, with representative values passed to it to
simulate the different instances of the notification.

We can't use the actual notification components in the stories due to
their data dependencies.

Release Notes:

- N/A

crates/collab_ui/src/notifications/collab_notification.rs

@@ -0,0 +1,52 @@
+use gpui::{img, prelude::*, AnyElement};
+use smallvec::SmallVec;
+use ui::prelude::*;
+
+#[derive(IntoElement)]
+pub struct CollabNotification {
+    avatar_uri: SharedString,
+    accept_button: Button,
+    dismiss_button: Button,
+    children: SmallVec<[AnyElement; 2]>,
+}
+
+impl CollabNotification {
+    pub fn new(
+        avatar_uri: impl Into<SharedString>,
+        accept_button: Button,
+        dismiss_button: Button,
+    ) -> Self {
+        Self {
+            avatar_uri: avatar_uri.into(),
+            accept_button,
+            dismiss_button,
+            children: SmallVec::new(),
+        }
+    }
+}
+
+impl ParentElement for CollabNotification {
+    fn children_mut(&mut self) -> &mut SmallVec<[AnyElement; 2]> {
+        &mut self.children
+    }
+}
+
+impl RenderOnce for CollabNotification {
+    fn render(self, cx: &mut WindowContext) -> impl IntoElement {
+        h_stack()
+            .text_ui()
+            .justify_between()
+            .size_full()
+            .overflow_hidden()
+            .elevation_3(cx)
+            .p_2()
+            .gap_2()
+            .child(img(self.avatar_uri).w_12().h_12().rounded_full())
+            .child(v_stack().overflow_hidden().children(self.children))
+            .child(
+                v_stack()
+                    .child(self.accept_button)
+                    .child(self.dismiss_button),
+            )
+    }
+}