Extract slash commands to their own crate (#23261)

This PR extracts the slash command definitions out of the `assistant`
crate and into their own `assistant_slash_commands` crate.

Release Notes:

- N/A

crates/assistant_slash_commands/src/now_command.rs

@@ -0,0 +1,69 @@
+use std::sync::atomic::AtomicBool;
+use std::sync::Arc;
+
+use anyhow::Result;
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandResult,
+};
+use chrono::Local;
+use gpui::{Task, WeakView};
+use language::{BufferSnapshot, LspAdapterDelegate};
+use ui::prelude::*;
+use workspace::Workspace;
+
+pub struct NowSlashCommand;
+
+impl SlashCommand for NowSlashCommand {
+    fn name(&self) -> String {
+        "now".into()
+    }
+
+    fn description(&self) -> String {
+        "Insert current date and time".into()
+    }
+
+    fn menu_text(&self) -> String {
+        self.description()
+    }
+
+    fn requires_argument(&self) -> bool {
+        false
+    }
+
+    fn complete_argument(
+        self: Arc<Self>,
+        _arguments: &[String],
+        _cancel: Arc<AtomicBool>,
+        _workspace: Option<WeakView<Workspace>>,
+        _cx: &mut WindowContext,
+    ) -> Task<Result<Vec<ArgumentCompletion>>> {
+        Task::ready(Ok(Vec::new()))
+    }
+
+    fn run(
+        self: Arc<Self>,
+        _arguments: &[String],
+        _context_slash_command_output_sections: &[SlashCommandOutputSection<language::Anchor>],
+        _context_buffer: BufferSnapshot,
+        _workspace: WeakView<Workspace>,
+        _delegate: Option<Arc<dyn LspAdapterDelegate>>,
+        _cx: &mut WindowContext,
+    ) -> Task<SlashCommandResult> {
+        let now = Local::now();
+        let text = format!("Today is {now}.", now = now.to_rfc2822());
+        let range = 0..text.len();
+
+        Task::ready(Ok(SlashCommandOutput {
+            text,
+            sections: vec![SlashCommandOutputSection {
+                range,
+                icon: IconName::CountdownTimer,
+                label: now.to_rfc2822().into(),
+                metadata: None,
+            }],
+            run_commands_in_text: false,
+        }
+        .to_event_stream()))
+    }
+}