javascript: Improve string literals handling

1. Don't include the newline itself in a line continuation construct.
   This fixes generation of e.g. properties with embedded line
   continuations.
2. Don't continue parsing strings past an unescaped newline (as naked
   newlines are invalid inside strings).  This avoids parsing the whole
   remaining file as a string in case of broken input.  It is both
   useful to better support partly written files and to avoid loading a
   whole malformed file in memory while reading it as a string.

See section 7.8.4 "String Literals" of ECMA-262:
http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf