feat(duplicates): add default_duplicate_prefix option

[n-crespo]

Exposes a config option to change the default text of "(duplicated) " shown when buffers/tabpages have no other way to be differentiated. Addresses #998.

return {
  "akinsho/bufferline.nvim",
  opts = {
    options = {
      default_duplicate_prefix = "(duplicated) ",
    }
  }
}