assistant: Extract ContextEditor and ContextHistory to their own modules (#23422)

This PR extracts the `ContextEditor` and `ContextHistory`
implementations into their own modules so that it's clearer which parts
depend on other constructs in the `assistant` crate.

Release Notes:

- N/A

crates/assistant/src/slash_command_picker.rs

@@ -5,7 +5,7 @@ use gpui::{AnyElement, DismissEvent, SharedString, Task, WeakView};
 use picker::{Picker, PickerDelegate, PickerEditorPosition};
 use ui::{prelude::*, ListItem, ListItemSpacing, PopoverMenu, PopoverTrigger, Tooltip};
 
-use crate::assistant_panel::ContextEditor;
+use crate::context_editor::ContextEditor;
 
 #[derive(IntoElement)]
 pub(super) struct SlashCommandSelector<T: PopoverTrigger> {