neovim: use vim.fn.getregionpos() to replace buffer_get_selection()

[fidgetingbits]

There is recently a new function in neovim called getregionpos which I believe will be better to use than the existing buffer_get_selection(). Also has the benefit of existing tests in neovim, etc. One thing I'm not sure about will be the 'reverse' part, but I suspect it will still be easier to just use vim.fn.getregionpos() and work out the reverse flag.

See https://github.com/vim/vim/pull/14617

We will have to wait at least for 0.11 release (October 31, 2024) for it to be available though.