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/assistant_slash_commands.rs

@@ -0,0 +1,57 @@
+mod auto_command;
+mod cargo_workspace_command;
+mod context_server_command;
+mod default_command;
+mod delta_command;
+mod diagnostics_command;
+mod docs_command;
+mod fetch_command;
+mod file_command;
+mod now_command;
+mod project_command;
+mod prompt_command;
+mod search_command;
+mod selection_command;
+mod streaming_example_command;
+mod symbols_command;
+mod tab_command;
+mod terminal_command;
+
+use gpui::AppContext;
+use language::{CodeLabel, HighlightId};
+use ui::ActiveTheme as _;
+
+pub use crate::auto_command::*;
+pub use crate::cargo_workspace_command::*;
+pub use crate::context_server_command::*;
+pub use crate::default_command::*;
+pub use crate::delta_command::*;
+pub use crate::diagnostics_command::*;
+pub use crate::docs_command::*;
+pub use crate::fetch_command::*;
+pub use crate::file_command::*;
+pub use crate::now_command::*;
+pub use crate::project_command::*;
+pub use crate::prompt_command::*;
+pub use crate::search_command::*;
+pub use crate::selection_command::*;
+pub use crate::streaming_example_command::*;
+pub use crate::symbols_command::*;
+pub use crate::tab_command::*;
+pub use crate::terminal_command::*;
+
+pub fn create_label_for_command(
+    command_name: &str,
+    arguments: &[&str],
+    cx: &AppContext,
+) -> CodeLabel {
+    let mut label = CodeLabel::default();
+    label.push_str(command_name, None);
+    label.push_str(" ", None);
+    label.push_str(
+        &arguments.join(" "),
+        cx.theme().syntax().highlight_id("comment").map(HighlightId),
+    );
+    label.filter_range = 0..command_name.len();
+    label
+}