fix #20575, syntax error for juxtaposing a string literal

[JeffBezanson]

No description provided.

[IainNZ]

Was this meant to also kill parse-time concatenation of two string literals? Its very useful for splitting long error messages over multiple lines without manually doing concatenation.

[IainNZ]

Python code but you get the point:
https://github.com/deepmind/sonnet/blob/31bc79cc9a6fe6e720e5c2402d68cc9cce2eb81c/sonnet/python/modules/conv.py#L176

On 0.6 the behaviour was

julia> x = "test""foo"
"testfoo"

julia> x = ("test"
       "foo")
ERROR: syntax: missing comma or ) in argument list

Which wasn't ideal due to the multiline case not working.