Introduce a new /delta command (#17903)

Release Notes:

- Added a new `/delta` command to re-insert changed files that were
previously included in a context.

---------

Co-authored-by: Roy <roy@anthropic.com>

crates/assistant_slash_command/Cargo.toml

@@ -19,4 +19,5 @@ gpui.workspace = true
 language.workspace = true
 parking_lot.workspace = true
 serde.workspace = true
+serde_json.workspace = true
 workspace.workspace = true