tab_switcher: Add support for tab switcher in assistant panel (#15475)

Additionally, I've generalized the implementation of tab switcher so
that - instead of explicitly listing panels it supports (at the time of
writing it was just the terminal panel and nothing else), it now relies
on Panel::pane trait method. As long as that's implemented, you get a
tab switcher support for free.

Release Notes:

- Added support for tab switcher in Assistant panel.

crates/assistant/src/assistant_panel.rs

@@ -686,13 +686,11 @@ impl AssistantPanel {
                 .focus_handle(cx)
                 .contains_focused(cx)
             {
-                if let Some(terminal_view) = terminal_panel
-                    .read(cx)
-                    .pane()
-                    .read(cx)
-                    .active_item()
-                    .and_then(|t| t.downcast::<TerminalView>())
-                {
+                if let Some(terminal_view) = terminal_panel.read(cx).pane().and_then(|pane| {
+                    pane.read(cx)
+                        .active_item()
+                        .and_then(|t| t.downcast::<TerminalView>())
+                }) {
                     return Some(InlineAssistTarget::Terminal(terminal_view));
                 }
             }