Implement Mega Stones as pairs {key: value}

[andrebastosdias]

Fixes #11683
Client PR (it is very very small): smogon/pokemon-showdown-client#2590
Implements Mega Stones as megaStone?: { [megaEvolves: string]: string } for normalization purposes.
If for whatever reason we want to keep megaEvolves, it can be extracted directly from the keys of megaStone.

[Slayer95]

My only concern is that an attacker can send a Pokémon with species constructor to the validator, and this key could be looked up in places such as

pokemon-showdown/data/rulesets.ts

Lines 1510 to 1512 in 7db6353

	if (item.megaStone?.[species.name]) {
	species = this.dex.species.get(item.megaStone[species.name]);
	typeTable = typeTable.filter(type => species.types.includes(type));