Add TrafficLights component (#3011)

This PR adds a `TrafficLights` component for GPUI2.

<img width="861" alt="Screenshot 2023-09-21 at 11 32 10 PM"
src="https://github.com/zed-industries/zed/assets/1486634/0fe0e847-49b3-44dc-bd4c-64f12f0051c1">

Release Notes:

- N/A

crates/ui/src/components/traffic_lights.rs

@@ -0,0 +1,30 @@
+use gpui2::elements::div;
+use gpui2::style::StyleHelpers;
+use gpui2::{Element, Hsla, IntoElement, ParentElement, ViewContext};
+
+use crate::theme;
+
+#[derive(Element)]
+pub struct TrafficLights {}
+
+pub fn traffic_lights() -> TrafficLights {
+    TrafficLights {}
+}
+
+impl TrafficLights {
+    fn render<V: 'static>(&mut self, _: &mut V, cx: &mut ViewContext<V>) -> impl IntoElement<V> {
+        let theme = theme(cx);
+
+        div()
+            .flex()
+            .items_center()
+            .gap_2()
+            .child(traffic_light(theme.lowest.negative.default.foreground))
+            .child(traffic_light(theme.lowest.warning.default.foreground))
+            .child(traffic_light(theme.lowest.positive.default.foreground))
+    }
+}
+
+fn traffic_light<V: 'static, C: Into<Hsla>>(fill: C) -> div::Div<V> {
+    div().w_3().h_3().rounded_full().fill(fill.into())
+}