Add new variables for the first and last braille page of the volume

[bertfrees]

These variables should be available at least in the context of the evaluate element.

The use case is for printing these values on the volume title page.

I'm not sure whether we need different variables for the first/last content page, and for the first/last page including pre/post-content. The respective variable names could be:

• volume-first-page
• volume-last-page
• volume-first-content-page
• volume-last-content-page