Extract ContextEditor to assistant_context_editor (#23433)

This PR extracts the `ContextEditor` to the `assistant_context_editor`
crate.

As part of this, we have decoupled the `ContextEditor` from the
`AssistantPanel`.

There is now an `AssistantPanelDelegate` that the `ContextEditor` uses
when it needs to interface with the Assistant panel.

Release Notes:

- N/A

crates/assistant/src/inline_assistant.rs

@@ -1,9 +1,8 @@
 use crate::{
-    humanize_token_count, AssistantPanel, AssistantPanelEvent, CycleNextInlineAssist,
-    CyclePreviousInlineAssist,
+    AssistantPanel, AssistantPanelEvent, CycleNextInlineAssist, CyclePreviousInlineAssist,
 };
 use anyhow::{anyhow, Context as _, Result};
-use assistant_context_editor::RequestType;
+use assistant_context_editor::{humanize_token_count, RequestType};
 use assistant_settings::AssistantSettings;
 use client::{telemetry::Telemetry, ErrorExt};
 use collections::{hash_map, HashMap, HashSet, VecDeque};