Always show large text files on _file split headers

Previously, large files with an \H file argument associated to them would show a message rather than the file contents both on their split and non-split versions, e.g.:

Now, the split version docs.ourbigbook.com/_file/index.js alwayws shows the full text file.

When not in split mode, limiting preview sizes is important otherwise multi-header pages might become far too big. Ideally we would have found a way to reliably use iframe + loading="lazy" to refer to the file without actually embedding it into the page as we do for images, but we haven't managed to do that so far.

This allows us to now see files that were previously not visible anywhere on the rendered HTML without download due to _raw files are downloaded rather than displayed in browser for certain file extensions on GitHub Pages.