fix(exit_when_last): 'exit_when_last' not working due to autocmd timing issue

[KineiChou]

When the exit_when_last option is set to true, closing the last main window should automatically trigger :qa to quit Neovim.

However, this feature is not currently working as expected. When the last main window is closed, Neovim does not exit; instead, the edgy sidebar windows remain.

The WinClosed event fires during the window closing process, not after the window has been completely removed from the window list.

Therefore, when check_main() callback executes, the window being closed still technically exists in Neovim's window list. This causes vim.tbl_isempty(wins.main) to return false, and the :qa exit logic is skipped.

Wrapping the check_main function call in vim.schedule() defers its execution to the 'next tick' of the Neovim event loop, by which point the WinClosed event has fully completed and the window has been removed from the list.