Added the ability to check the number of manuscript sheets

Hi. I've been writing well since before the update.

It may not be very important for web novel work, but I would like to be able to count the number of words on a 200-character manuscript paper in addition to the word count (as shown in the post below).

I'm always transferring the text to another software to check the count, and it would be handy to be able to do it easily in Novella.