Add assistant_context_editor crate (#23429)

This PR adds a new `assistant_context_editor` crate.

This will ultimately house the `ContextEditor` so that it can be
consumed by both `assistant` and `assistant2`.

For the purposes of this PR, we just introduce the crate and move some
supporting constructs to it, such as the `ContextStore`.

Release Notes:

- N/A

crates/assistant/src/inline_assistant.rs

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