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_commands/src/assistant_slash_commands.rs

@@ -12,11 +12,6 @@ mod selection_command;
 mod streaming_example_command;
 mod symbols_command;
 mod tab_command;
-mod terminal_command;
-
-use gpui::App;
-use language::{CodeLabel, HighlightId};
-use ui::ActiveTheme as _;
 
 pub use crate::cargo_workspace_command::*;
 pub use crate::context_server_command::*;
@@ -32,16 +27,5 @@ 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: &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
-}
+use assistant_slash_command::create_label_for_command;