Add missing FRLG sprites of Pokémon from Gen II and III

[Delgan]

Hi.

Some firered-leafgreen sprites were added back in 2020 by #19, but it seems to have only added Pokémon from the first generation. Sprites for Pokémon from generations II and III were missing. This PR aims to complete them.

I used Bulbapedia as the canonical source for the sprites. I expected it to be straightforward, but I fell down a rabbit hole because of inconsistencies in the color palettes...

If we look at the history on Bulbapedia, we can see sprites were changed for the reason "fixed colors":

I do not know the underlying technical reason that triggered this update. Note that PokeAPI currently uses the previous sprite version for the 1st-gen sprites. Since I used the latest version for the new sprites, I also updated the 1st-gen sprites to maintain colors palette consistency. This resulted in two commits, best viewed and merged separately.

I also wanted to make sure that Bulbapedia's colors were as accurate as possible. So I compared different sources. But I couldn't come to a conclusion. It's hard to know which one is "right". I'll have to look into this further in the future...

I feel the latest Bulbapedia's sprites are a bit over-exposed, but they match the pokefirered decompiled ones, although no tested emulator matches. Yet, I think the best approach is still to use Bulbapedia as the authoritative source of truth. Therefore, I will likely open another PR to update the ruby-sapphire and emerald sprites as well to ensure consistency with firered-leafgreen.

[Delgan]

To clarify: qualifying the sprites as "missing" is a bit misleading, because they are actually the same as for ruby-sapphire, which is surely the reason they were not added in the first place.

[Naramsim]

The changes are almost no noticeable. Let's merge this in anyway since we trust Bulbapedia.