fix: replace BufModifiedSet with OptionSet for Neovim 0.11 compat

[iqrar-eng]

BufModifiedSet was removed in Neovim 0.11. Replacing with OptionSet pattern=modified which has been available since Vim 7.4.729 so no version guard is needed.

[Hajime-Suzuki]

Thank you so much for the PR! Migrating off BufModifiedSet is the right move for Neovim 0.13.

One correction: BufModifiedSet wasn't removed in 0.11. It still works on stable 0.11/0.12. Removal is in neovim#35610 and only affects 0.13 nightly today.

Also, OptionSet modified only became reliable in 0.13 (same PR). On 0.11/0.12 it doesn't fire on edits, so this change would break modified-icon updates for stable users.

Could we keep a version guard?

if vim.fn.has("nvim-0.13") == 1 then
  -- OptionSet modified
else
  -- BufModifiedSet
end

Same approach as nvim-tree#3325 and barbar#668.

[iqrar-eng]

Thanks for the clarification — and thank you very much for vuffers.nvim!

The version guard approach (vim.fn.has("nvim-0.13")) turned out to be unreliable — BufModifiedSet was invalid even when the check passed. Switched to checking if the event actually exists at runtime instead:

if vim.fn.exists("##BufModifiedSet") == 1 then
  vim.api.nvim_create_autocmd({ "BufModifiedSet" }, {
    pattern = "*",
    group = constants.AUTO_CMD_GROUP,
    callback = function(buffer)
      if not buf_utils.is_valid_buf(buffer) then
        return
      end
      logger.debug("BufModifiedSet", { buffer = buffer })
      ui.update_modified_icon(buffer)
    end,
  })
else
  vim.api.nvim_create_autocmd({ "OptionSet" }, {
    pattern = "modified",
    group = constants.AUTO_CMD_GROUP,
    callback = function(buffer)
      if not buf_utils.is_valid_buf(buffer) then
        return
      end
      logger.debug("OptionSet modified", { buffer = buffer })
      ui.update_modified_icon(buffer)
    end,
  })
end

vim.fn.exists("##BufModifiedSet") returns 1 only if the event is supported by the running Neovim binary, so no version guessing needed. Let me know if anything needs adjusting.

[Hajime-Suzuki]

Thank you so much for your contribution! 🙏