The previous attempt to support newlines after pipes changed the lexer to
swallow newlines after encountering a pipe. This has two problems that are
difficult to fix:
1. comments cannot be placed after the pipe
2. fish_indent won't know about the newlines, so it will erase them
Address these problems by removing the lexer behavior, and replacing it
with a new parser symbol "optional_newlines" allowing the newlines to be
reflected directly in the fish grammar.
This was a symbol that represented either an argument or a redirection.
This was only used as part of argument_or_redirection_list.
It's simpler to just have these types be alternatives in the list type.
