Handle quoted heredoc delimiter names

[rcjsuen]

RUN <<'EOT'
  echo
EOT
RUN echo

const lib = require("dockerfile-ast");
const dockerfile = lib.DockerfileParser.parse("RUN <<'EOT'\n  echo\nEOT\nRUN echo");
console.log(dockerfile.getInstructions().length);

1

It should print 2 instead of 1.