Add support for type="image/x.djvu" to processEmbeds in the extension.

[ichbinjakes]

I cam across a page that embedded djvu content with type="image/x.djvu" . Need to change Line 58 in content.js:

        const embeds = document.querySelectorAll('embed[type="image/x-djvu"], embed[type="image/vnd.djvu"], embed[type="image/x.djvu"]');