[Developer Request] Publicize `necronomicon`'s internals

[realSquidCoder]

Make necronomicon's internal functions return strings rather than printing directly and make necronomicon a module or similar so one can call them rather than copying them. I think this would enable someone to make #5234 a thing.

Here is my rough mock up of the necronomicon notification, for the sake of getting someone started

local necro = require('necronomicon') -- maybe needs to be reqscript instead

[...]

{
        name='necronomicon',
        desc='Notifies you to books in your fort containing the secrets of life and death.',
        default=true,
        dwarf_fn=function ()
            for _, vec in ipairs{df.global.world.items.other.BOOK, df.global.world.items.other.TOOL} do
                for _, item in ipairs(vec) do
                    local _, interactions = necro.get_book_interactions(item)
                    if next(interactions) ~= nil then
                        return {{text='Necromancy Warning!', pen=COLOR_LIGHTRED}}
                    end
                end
            end
        end,
        on_click=function ()
            local results = {}
            for _, vec in ipairs{df.global.world.items.other.BOOK, df.global.world.items.other.TOOL} do
                for _, item in ipairs(vec) do
                    local title, interactions = necro.get_book_interactions(item)
                    if next(interactions) ~= nil then
                        results.append({("\nBook: " .. dfhack.df2console(title)), necro.print_interactions(interactions)})
                    end
                end
            end
            for _, item in ipairs(df.global.world.items.other.SLAB) do
                if check_slab_secrets(item) then
                    local artifact = get_item_artifact(item)
                    local name = dfhack.translation.translateName(artifact.name)
                    results.append({("\nSlab: " .. dfhack.df2console(name))})
                end
            end
            if results == {} then
                results = {'No necromactic books, scrolls, or slabs were found'}
            end
            local msg = 'Some necromactic texts have been found in your fort:\n\n' .. results
            dlg.showMessage("Necromancy Warning!", msg, COLOR_WHITE)
        end
    },