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/terminal_inline_assistant.rs

@@ -1,7 +1,6 @@
-use crate::{
-    humanize_token_count, AssistantPanel, AssistantPanelEvent, RequestType, DEFAULT_CONTEXT_LINES,
-};
+use crate::{humanize_token_count, AssistantPanel, AssistantPanelEvent, DEFAULT_CONTEXT_LINES};
 use anyhow::{Context as _, Result};
+use assistant_context_editor::RequestType;
 use assistant_settings::AssistantSettings;
 use client::telemetry::Telemetry;
 use collections::{HashMap, VecDeque};