assistant: Add /now slash command (#12856)

This PR adds a `/now` command to the Assistant for indicating the
current date and time to the model.

Release Notes:

- Added `/now` command to the Assistant for getting the current date and
time.

crates/assistant/src/slash_command/now_command.rs

@@ -0,0 +1,83 @@
+use std::sync::atomic::AtomicBool;
+use std::sync::Arc;
+
+use anyhow::Result;
+use assistant_slash_command::{SlashCommand, SlashCommandOutput, SlashCommandOutputSection};
+use chrono::{DateTime, Local};
+use gpui::{AppContext, Task, WeakView};
+use language::LspAdapterDelegate;
+use ui::{prelude::*, ButtonLike, ElevationIndex};
+use workspace::Workspace;
+
+pub(crate) struct NowSlashCommand;
+
+impl SlashCommand for NowSlashCommand {
+    fn name(&self) -> String {
+        "now".into()
+    }
+
+    fn description(&self) -> String {
+        "insert the current date and time".into()
+    }
+
+    fn menu_text(&self) -> String {
+        "Insert current date and time".into()
+    }
+
+    fn requires_argument(&self) -> bool {
+        false
+    }
+
+    fn complete_argument(
+        &self,
+        _query: String,
+        _cancel: Arc<AtomicBool>,
+        _workspace: Option<WeakView<Workspace>>,
+        _cx: &mut AppContext,
+    ) -> Task<Result<Vec<String>>> {
+        Task::ready(Ok(Vec::new()))
+    }
+
+    fn run(
+        self: Arc<Self>,
+        _argument: Option<&str>,
+        _workspace: WeakView<Workspace>,
+        _delegate: Arc<dyn LspAdapterDelegate>,
+        _cx: &mut WindowContext,
+    ) -> Task<Result<SlashCommandOutput>> {
+        let now = Local::now();
+        let text = format!("Today is {now}.", now = now.to_rfc3339());
+        let range = 0..text.len();
+
+        Task::ready(Ok(SlashCommandOutput {
+            text,
+            sections: vec![SlashCommandOutputSection {
+                range,
+                render_placeholder: Arc::new(move |id, unfold, _cx| {
+                    NowPlaceholder { id, unfold, now }.into_any_element()
+                }),
+            }],
+            run_commands_in_text: false,
+        }))
+    }
+}
+
+#[derive(IntoElement)]
+struct NowPlaceholder {
+    pub id: ElementId,
+    pub unfold: Arc<dyn Fn(&mut WindowContext)>,
+    pub now: DateTime<Local>,
+}
+
+impl RenderOnce for NowPlaceholder {
+    fn render(self, _cx: &mut WindowContext) -> impl IntoElement {
+        let unfold = self.unfold;
+
+        ButtonLike::new(self.id)
+            .style(ButtonStyle::Filled)
+            .layer(ElevationIndex::ElevatedSurface)
+            .child(Icon::new(IconName::CountdownTimer))
+            .child(Label::new(self.now.to_rfc3339()))
+            .on_click(move |_, cx| unfold(cx))
+    }
+}