Fix diag messages for magics

[rchiodo]

Fixes: #17058

See associated PR:
microsoft/vscode-jupyter-lsp-middleware#4

[deepbIue]

It appears that the implementation does not handle magic commands that are indented.
Example:

def a():
    pass

def b():
    #!%timeit a  # this does not work
    %timeit a  # this works but raises a syntax error

b()
#!%timeit a  # this works and does not raise a syntax error