Reexport arg_split on emscripten.

Closes jupyterlite/demo#139

IPython/utils/_process_emscripten.py

@@ -1,21 +1,23 b''
 """Emscripten-specific implementation of process utilities.
 
 This file is only meant to be imported by process.py, not by end-users.
 """
 
 
+from ._process_common import arg_split
+
+
 def system(cmd):
     raise OSError("Not available")
 
 
 def getoutput(cmd):
     raise OSError("Not available")
 
 
 def check_pid(cmd):
     raise OSError("Not available")
 
 
-def arg_split(s, posix=False, strict=True):
-    """This one could be made to work but it's not clear if it would be useful..."""
-    raise OSError("Not available")
+# `arg_split` is still used by magics regardless of whether we are on a posix/windows/emscipten
+__all__ = ["system", "getoutput", "check_pid", "arg_split"]