[Feature Request]: Expose next_item API

[dennypenta]

What?

Expose next_item api, so as user could get a next buffer a potential call of cycle in a given direction or BufferLineCyclePrev cmd move to.

Why?

In order to easier integrate plugins such as bufdel.
At the moment a user is required to call a private API (I guess so) and duplicate a part of cycle command, e.g.

    {
      "ojroques/nvim-bufdel",
      opts = {
        next = function()
          -- TODO: if it's first then next, otherwise prev
          local state = require("bufferline.lazy").require("bufferline.state")
          local index = require("bufferline.commands").get_current_element_index(state)
          if not index then
            return
          end

          local next_index = index - 1

          if next_index <= 0 then
            next_index = #state.components
          end

          local item = state.components[next_index]
          if not item then
            return vim.notify("This " .. item.type .. " does not exist", vim.log.levels.ERROR)
          end
          return item.id
        end,
        quit = true,
      },
    },