[Modding/Dev] Separate books.lua definitions into a bookdefs.lua

[Friendly Grass]

In prefabs/books.lua, book data definitions are described in a table `book_defs` which is then used at the end of the file to generate prefabs for each book. This table could be factored out of books.lua into a separate file akin to wx78_moduledefs.lua, battlesongsdefs.lua, etc. This way mods can simply require bookdefs and modify the table - tweaking books and adding new ones - before books.lua actually uses it to construct prefabs.