Fix parsing error with heredocs within command chains with escaped
newlines.

When chaining commands in a `RUN` instructions, it is possible that one
of the middle commands takes a heredoc. It is often convenient to also
break up the resulting long line with escapes, e.g.:

```Dockerfile
RUN foo \
 && bar <<EOF >> bar.txt \
 && baz
inside heredoc
content
EOF
```

The heredoc parser should not fail on escaped newlines in this command
chain.
This requires two modifications:

  - Parsing the rest of the command chain, after a heredoc should not
    end on the literal '\n' character, but should be done with the
    `untilEol` parser, which respects the escaped newlins
  - Parsing the command chain before the heredoc requires casting
    escaped newlines to spaces as to not fail the parser on a '\n'
    character

This approach works with the caveat that the command chain after the
heredoc is ignored and will not end up in the parsed text.

fixes: hadolint/hadolint#923
Signed-off-by: Moritz Röhrich <[email protected]>