Allow {parent} to point to {file} header

= my/file.txt

= Asdf
{parent=my/file.txt}
{parentFile}

Same for tags.

Currently there is some confusion in the code on treating the <>{file} like the file in = Header{file}: one if about pointing to things, the other is about the current thing. We will disambiguate with parentFile.

Same for tag and tagFile.