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/tab.rs

@@ -0,0 +1,56 @@
+use gpui2::elements::div;
+use gpui2::style::{StyleHelpers, Styleable};
+use gpui2::{Element, IntoElement, ParentElement, ViewContext};
+
+use crate::theme;
+
+#[derive(Element)]
+pub struct Tab {
+    title: &'static str,
+    enabled: bool,
+}
+
+pub fn tab<V: 'static>(title: &'static str, enabled: bool) -> impl Element<V> {
+    Tab { title, enabled }
+}
+
+impl Tab {
+    fn render<V: 'static>(&mut self, _: &mut V, cx: &mut ViewContext<V>) -> impl IntoElement<V> {
+        let theme = theme(cx);
+
+        div()
+            .px_2()
+            .py_0p5()
+            .flex()
+            .items_center()
+            .justify_center()
+            .rounded_lg()
+            .fill(if self.enabled {
+                theme.highest.on.default.background
+            } else {
+                theme.highest.base.default.background
+            })
+            .hover()
+            .fill(if self.enabled {
+                theme.highest.on.hovered.background
+            } else {
+                theme.highest.base.hovered.background
+            })
+            .active()
+            .fill(if self.enabled {
+                theme.highest.on.pressed.background
+            } else {
+                theme.highest.base.pressed.background
+            })
+            .child(
+                div()
+                    .text_sm()
+                    .text_color(if self.enabled {
+                        theme.highest.base.default.foreground
+                    } else {
+                        theme.highest.variant.default.foreground
+                    })
+                    .child(self.title),
+            )
+    }
+}