Allow panel serve of notebook that starts with magics

[droumis]

Panel won't serve a notebook that starts with something like:

%load_ext autoreload
%autoreload 2

error:

Invalid syntax in '<file>.ipynb' on line 4:
   "cell_type": "code",

Traceback (most recent call last):
  File "<...>/bokeh/application/handlers/code_runner.py", line 104, in __init__
    nodes = ast.parse(source, os.fspath(path))
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<...lib>/python3.12/ast.py", line 52, in parse
    return compile(source, filename, mode, flags,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<...file>.ipynb", line 4
    "cell_type": "code",
SyntaxError: invalid syntax