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/Cargo.toml

@@ -22,14 +22,13 @@ test-support = [
 
 [dependencies]
 anyhow.workspace = true
+assistant_context_editor.workspace = true
 assistant_settings.workspace = true
 assistant_slash_command.workspace = true
 assistant_slash_commands.workspace = true
 assistant_tool.workspace = true
 async-watch.workspace = true
-chrono.workspace = true
 client.workspace = true
-clock.workspace = true
 collections.workspace = true
 command_palette_hooks.workspace = true
 context_server.workspace = true
@@ -50,27 +49,21 @@ log.workspace = true
 lsp.workspace = true
 menu.workspace = true
 multi_buffer.workspace = true
-open_ai = { workspace = true, features = ["schemars"] }
 parking_lot.workspace = true
 paths.workspace = true
 picker.workspace = true
 project.workspace = true
 prompt_library.workspace = true
 proto.workspace = true
-regex.workspace = true
 rope.workspace = true
-rpc.workspace = true
 schemars.workspace = true
 search.workspace = true
 semantic_index.workspace = true
 serde.workspace = true
-serde_json.workspace = true
 settings.workspace = true
 similar.workspace = true
-smallvec.workspace = true
 smol.workspace = true
 streaming_diff.workspace = true
-strum.workspace = true
 telemetry.workspace = true
 telemetry_events.workspace = true
 terminal.workspace = true
@@ -79,7 +72,6 @@ text.workspace = true
 theme.workspace = true
 ui.workspace = true
 util.workspace = true
-uuid.workspace = true
 workspace.workspace = true
 zed_actions.workspace = true