Add an explore extensions button to welcome screen (#11265)

We are still getting comments from users saying they didn't know we had
extensions until just now. This doesn't fix anything for existing users,
but it helps make new users more aware, earlier on. There's probably
more we need to do for discoverability, but this feels like it should
help.

Release Notes:

- N/A

Cargo.lock

@@ -11797,6 +11797,7 @@ dependencies = [
  "copilot_ui",
  "db",
  "editor",
+ "extensions_ui",
  "fuzzy",
  "gpui",
  "install_cli",

crates/welcome/Cargo.toml

@@ -19,6 +19,7 @@ anyhow.workspace = true
 client.workspace = true
 copilot_ui.workspace = true
 db.workspace = true
+extensions_ui.workspace = true
 fuzzy.workspace = true
 gpui.workspace = true
 install_cli.workspace = true

crates/welcome/src/welcome.rs

@@ -145,6 +145,16 @@ impl Render for WelcomePage {
                                         );
                                         copilot_ui::initiate_sign_in(cx);
                                     })),
+                            )
+                            .child(
+                                Button::new("explore extensions", "Explore extensions")
+                                    .full_width()
+                                    .on_click(cx.listener(|this, _, cx| {
+                                        this.telemetry.report_app_event(
+                                            "welcome page: open extensions".to_string(),
+                                        );
+                                        cx.dispatch_action(Box::new(extensions_ui::Extensions));
+                                    })),
                             ),
                     )
                     .child(