Give Pokémon internal IDs their own table and add all gens' IDs.

- Gen I has them all mixed around.
- Gen II has no surprises, but I figured it's good to be thorough.
- Gen III has the first 251 in order, then a big break, then the
  third-gen Pokémon mixed around, though families are usually together.
- Gen IV has the 493 in order and then alternate forms after Arceus,
  which will be useful to have once Gen V comes and we have to bump
  the alt forms in the pokemon table forward.

pokedex/data/csv/pokedex_version_groups.csv

@@ -9,5 +9,3 @@ pokedex_id,version_group_id
 5,8
 6,9
 7,10
-8,1
-8,2