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_context_editor/src/context_editor.rs

@@ -1,3 +1,6 @@
+use crate::language_model_selector::{
+    LanguageModelSelector, LanguageModelSelectorPopoverMenu, ToggleModelSelector,
+};
 use anyhow::Result;
 use assistant_settings::AssistantSettings;
 use assistant_slash_command::{SlashCommand, SlashCommandOutputSection, SlashCommandWorkingSet};
@@ -36,9 +39,6 @@ use language_model::{
     LanguageModelImage, LanguageModelProvider, LanguageModelProviderTosView, LanguageModelRegistry,
     Role,
 };
-use language_model_selector::{
-    LanguageModelSelector, LanguageModelSelectorPopoverMenu, ToggleModelSelector,
-};
 use multi_buffer::MultiBufferRow;
 use picker::Picker;
 use project::{Project, Worktree};