Implement automatic time-ordering of coordinates

[malcolmbarrett]

This PR expands time_ordered_coords() to be able to create time-ordered coordinates automatically, e.g.

library(ggdag)
dagify(
  z3 ~ y,
  y ~ x1 + x2,
  a ~ z1 + z2 + z3,
  coords = time_ordered_coords()
) %>% ggdag()

^{Created on 2023-08-07 with reprex v2.0.2}

You can also delay this step to tidy_dagitty() and specify "time_ordered" as the layout