assistant: Feature flag terminal inline assistant (#13732)

This PR adds a feature flag for the terminal inline assistant because we
want to keep it internal for now.

Release Notes:

- N/A

crates/assistant/Cargo.toml

@@ -23,6 +23,7 @@ client.workspace = true
 collections.workspace = true
 command_palette_hooks.workspace = true
 editor.workspace = true
+feature_flags.workspace = true
 file_icons.workspace = true
 fs.workspace = true
 futures.workspace = true