cli[minor]: Add first version of migrate (#20902)

Adds a first version of the migrate script.

libs/cli/langchain_cli/cli.py

@@ -1,3 +1,4 @@
+import importlib
 from typing import Optional
 
 import typer
@@ -22,6 +23,13 @@ app.add_typer(
 )
 
 
+# If libcst is installed, add the migrate namespace
+if importlib.util.find_spec("libcst"):
+    from langchain_cli.namespaces.migrate import main as migrate_namespace
+
+    app.add_typer(migrate_namespace.app, name="migrate", help=migrate_namespace.__doc__)
+
+
 def version_callback(show_version: bool) -> None:
     if show_version:
         typer.echo(f"langchain-cli {__version__}")