feat: Custom open file handler

[mikew]

This allows people to implement their own handling when the $EDITOR script is called.

---@param filename string
---@param line number?
---@param col number?
_G.nvrh_open_file_handler = function(filename, line, col)
  vim.cmd.vsplit(filename)

  if line ~= nil then
    local window = vim.api.nvim_get_current_win()
    -- Note that `col` may be nil even though `line` is set.
    pcall(vim.api.nvim_win_set_cursor, window, { line, col or 0 })
  end
end