(assets): Add the majority of official new mega evolution shiny forms

[JoseBaGra]

This adds most of the new shiny mega sprites to the official folder. This is a repalette made with my humble image-editing skills. If anyone has better skills, feel free to update them or replace them.

[Naramsim]

Hi, are you saying these aren't the official images released by Pokemon.com? It's an editing of yours? I'm not sure if merging this PR would defy the official artwork statement.

[JoseBaGra]

The images are official, but I edited them to have the colors from shinies based on the ones that you can see in the games. The Pokémon Company rarely releases official art of its shiny Pokémon.
In the readme, it says:

We thank @DevMike123 and Pokétwo who are the spriters that created these custom shiny sprites.

So I supposed that I could recolor the official ones with the shiny colors to have them.