Splitting a Command Line Like a Shell

Implement shell_split(line) (like shlex.split, without using it) that splits a command line into arguments:

• whitespace separates arguments
• inside single quotes everything is literal
• inside double quotes, a backslash escapes " and \ (other backslashes stay as-is)
• outside quotes, a backslash escapes the next character
• quotes can be glued to other text: a"b c"d is one argument ab cd; "" is an empty argument
• an unclosed quote or a trailing backslash raises ValueError