feat: implement shiny chance

[koffydrop]

Implemented a default random shiny chance that can be overridden to force a shiny or normal with --shiny true|false (which defaults to true if not specified), and to change the rate with --shiny-rate <number>.

[talwat]

I feel like adding options for this is a bit redundant. Perhaps replacing these options with environment variables could work? For example, POKEGET_SHINY_RATE. I also do not understand why shiny was made into a Option<bool>.

The benefit with environment variables is that they don't need to clutter the help screen or the main clap struct.

[koffydrop]

Shiny being made into an Option<bool> is so that if it's None, the the shiny chance takes effect, if it's specified without a value it defaults to true, and you can pass a truthy or falsy value still. It makes sense that way in my head but I'm open to change it (or if there's a better way of implementing it).

I can definitely see the rate being cluttered and look into making it an environment variable though.