Upsell built-in features on the extensions page (#14516)

This PR extends the extensions page with support for upselling built-in
Zed features when certain keywords are searched for.

This should help inform users about features that Zed has out-of-the-box
when they go looking for them as extensions.

For example, when someone searches "vim":

<img width="1341" alt="Screenshot 2024-07-15 at 4 58 44 PM"
src="https://github.com/user-attachments/assets/b256d07a-559a-43c2-b491-3eca5bff436e">

Here are more examples of what the upsells can look like:

<img width="1341" alt="Screenshot 2024-07-15 at 4 54 39 PM"
src="https://github.com/user-attachments/assets/1f453132-ac14-4884-afc4-7c12db47ad1d">

Release Notes:

- Added banners for built-in Zed features when corresponding keywords
are used in the extension search.

crates/extensions_ui/src/components/feature_upsell.rs

@@ -0,0 +1,72 @@
+use gpui::{AnyElement, Div, StyleRefinement};
+use smallvec::SmallVec;
+use ui::{prelude::*, ButtonLike};
+
+#[derive(IntoElement)]
+pub struct FeatureUpsell {
+    base: Div,
+    text: SharedString,
+    docs_url: Option<SharedString>,
+    children: SmallVec<[AnyElement; 2]>,
+}
+
+impl FeatureUpsell {
+    pub fn new(text: impl Into<SharedString>) -> Self {
+        Self {
+            base: h_flex(),
+            text: text.into(),
+            docs_url: None,
+            children: SmallVec::new(),
+        }
+    }
+
+    pub fn docs_url(mut self, docs_url: impl Into<SharedString>) -> Self {
+        self.docs_url = Some(docs_url.into());
+        self
+    }
+}
+
+impl ParentElement for FeatureUpsell {
+    fn extend(&mut self, elements: impl IntoIterator<Item = AnyElement>) {
+        self.children.extend(elements)
+    }
+}
+
+// Style methods.
+impl FeatureUpsell {
+    fn style(&mut self) -> &mut StyleRefinement {
+        self.base.style()
+    }
+
+    gpui::border_style_methods!({
+        visibility: pub
+    });
+}
+
+impl RenderOnce for FeatureUpsell {
+    fn render(self, cx: &mut WindowContext) -> impl IntoElement {
+        self.base
+            .p_4()
+            .justify_between()
+            .border_color(cx.theme().colors().border)
+            .child(v_flex().overflow_hidden().child(Label::new(self.text)))
+            .child(h_flex().gap_2().children(self.children).when_some(
+                self.docs_url,
+                |el, docs_url| {
+                    el.child(
+                        ButtonLike::new("open_docs")
+                            .child(
+                                h_flex()
+                                    .gap_2()
+                                    .child(Label::new("View docs"))
+                                    .child(Icon::new(IconName::ArrowUpRight)),
+                            )
+                            .on_click({
+                                let docs_url = docs_url.clone();
+                                move |_event, cx| cx.open_url(&docs_url)
+                            }),
+                    )
+                },
+            ))
+    }
+}