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/collab/src/tests/integration_tests.rs

@@ -6,7 +6,7 @@ use crate::{
     },
 };
 use anyhow::{anyhow, Result};
-use assistant::ContextStore;
+use assistant_context_editor::ContextStore;
 use assistant_slash_command::SlashCommandWorkingSet;
 use assistant_tool::ToolWorkingSet;
 use call::{room, ActiveCall, ParticipantLocation, Room};