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_context_editor/Cargo.toml

@@ -0,0 +1,58 @@
+[package]
+name = "assistant_context_editor"
+version = "0.1.0"
+edition.workspace = true
+publish.workspace = true
+license = "GPL-3.0-or-later"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/assistant_context_editor.rs"
+
+[dependencies]
+anyhow.workspace = true
+assistant_slash_command.workspace = true
+assistant_slash_commands.workspace = true
+assistant_tool.workspace = true
+chrono.workspace = true
+client.workspace = true
+clock.workspace = true
+collections.workspace = true
+context_server.workspace = true
+editor.workspace = true
+feature_flags.workspace = true
+fs.workspace = true
+futures.workspace = true
+fuzzy.workspace = true
+gpui.workspace = true
+language.workspace = true
+language_model.workspace = true
+language_models.workspace = true
+log.workspace = true
+open_ai.workspace = true
+paths.workspace = true
+project.workspace = true
+prompt_library.workspace = true
+regex.workspace = true
+rpc.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+smallvec.workspace = true
+smol.workspace = true
+strum.workspace = true
+telemetry_events.workspace = true
+text.workspace = true
+ui.workspace = true
+util.workspace = true
+uuid.workspace = true
+
+[dev-dependencies]
+language_model = { workspace = true, features = ["test-support"] }
+parking_lot.workspace = true
+pretty_assertions.workspace = true
+rand.workspace = true
+settings.workspace = true
+unindent.workspace = true
+workspace = { workspace = true, features = ["test-support"] }