Add ".swx" suffix to file watcher's ignore list

Add ".swx" suffix to file watcher's ignore list

When a user uses Vim to open a file, it briefly creates a
".filename.md.swx" file which needlessly triggers a website rebuild.

According to a comment in Vim's src/memline.c:

    /*
     * on MS-DOS compatible filesystems (e.g. messydos) file.doc.swp
     * and file.doc are the same file. To guess if this problem is
     * present try if file.doc.swx exists. If it does, we set
     * buf->b_shortname and try file_doc.swp (dots replaced by
     * underscores for this file), and try again. If it doesn't we
     * assume that "file.doc.swp" already exists.
     */