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/assistant_panel.rs

@@ -4,11 +4,11 @@ use crate::context_editor::{
 use crate::context_history::ContextHistory;
 use crate::{
     slash_command::SlashCommandCompletionProvider,
-    terminal_inline_assistant::TerminalInlineAssistant, Context, ContextId, ContextStore,
-    ContextStoreEvent, DeployHistory, DeployPromptLibrary, InlineAssistant, InsertDraggedFiles,
-    NewContext, ToggleFocus, ToggleModelSelector,
+    terminal_inline_assistant::TerminalInlineAssistant, DeployHistory, DeployPromptLibrary,
+    InlineAssistant, InsertDraggedFiles, NewContext, ToggleFocus, ToggleModelSelector,
 };
 use anyhow::Result;
+use assistant_context_editor::{Context, ContextId, ContextStore, ContextStoreEvent};
 use assistant_settings::{AssistantDockPosition, AssistantSettings};
 use assistant_slash_command::SlashCommandWorkingSet;
 use assistant_tool::ToolWorkingSet;