chore: Make terminal_view own the TerminalSlashCommand (#31070)

This reduces 'touch crates/editor/src/editor.rs && cargo +nightly build'
from 8.9s to 8.5s. That same scenario used to take 8s less than a week
ago. :)
I'm measuring with nightly rustc, because it's compile times are better
than those of stable thanks to
https://github.com/rust-lang/rust/pull/138522

main (8.2s total):

![image](https://github.com/user-attachments/assets/767a2ac4-7bba-4147-bd16-9b09eed5b433)

[cargo-timing.html.zip](https://github.com/user-attachments/files/20364175/cargo-timing.html.zip)

#22be776 (7.5s total):

[cargo-timing-20250521T085303.892834Z.html.zip](https://github.com/user-attachments/files/20364391/cargo-timing-20250521T085303.892834Z.html.zip)

![image](https://github.com/user-attachments/assets/c4476df9-cb6e-4403-b0db-de00521f1fd0)


Release Notes:

- N/A

crates/assistant_slash_command/src/assistant_slash_command.rs

@@ -9,6 +9,7 @@ use anyhow::Result;
 use futures::StreamExt;
 use futures::stream::{self, BoxStream};
 use gpui::{App, SharedString, Task, WeakEntity, Window};
+use language::HighlightId;
 use language::{BufferSnapshot, CodeLabel, LspAdapterDelegate, OffsetRangeExt};
 pub use language_model::Role;
 use serde::{Deserialize, Serialize};
@@ -16,6 +17,7 @@ use std::{
     ops::Range,
     sync::{Arc, atomic::AtomicBool},
 };
+use ui::ActiveTheme;
 use workspace::{Workspace, ui::IconName};
 
 pub fn init(cx: &mut App) {
@@ -325,6 +327,18 @@ impl SlashCommandLine {
     }
 }
 
+pub fn create_label_for_command(command_name: &str, arguments: &[&str], cx: &App) -> 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
+}
+
 #[cfg(test)]
 mod tests {
     use pretty_assertions::assert_eq;