Add past abilities and stats for pokemon endpoints

[jemarq04]

This PR does the following:

• Adds past_abilities and past_stats fields to the Pokemon model
• Creates PokemonAbilityPast and PokemonStatPast structs to hold information for past abilities and stats
  • The documentation comments were taken from PokeAPI. The past stats information is still in an active PR (Add Past Stats to documentation PokeAPI/pokeapi.co#173) but I expect it to be approved without issue.
• Modified the ability field for PokemonAbility to be Option<_>, since past_abilities can hold null abilities (i.e. if the ability slot was empty prior to a certain generation).

The static resources was re-created and all examples passed without issue.

[jemarq04]

I should note that the hyperlink to the PokemonStatPast documentation won't lead anywhere yet until the PR I linked is approved - this can always wait until then to be sure the link is correct.