Fix multi-line-string-body grammar

[brandonchinn178]

multi-line-raw-string-body is already non-greedy:

multi-line-raw-string-body :=
    (unicode - disallowed-literal-code-points)*?

The grammar seems incorrect if it's greedy, since it'll always consume the newline following it:

quoted-string :=
    '"' single-line-string-body '"' |
    '"""' newline
    (multi-line-string-body newline)?
    (unicode-space | ws-escape)* '"""'

Furthermore, the grammar currently always requires string-character after " or "", which doesn't allow this value:

"""
""
"""