[Tools] Include o3de-extras in script

[ShaunaGordon]

Currently, o3de-extras is kind of an afterthought that requires copying the gems into the main o3de Gems folder, then running that. We can do better.

There are two possible ways to go about this:

1. Make a new script that handles o3de-extras on its own.
2. Roll it into the Gems script.

Ideally, I'd prefer Option 2, because it's one less manual (or pseudo-manual, once it's fully automated) step, but I'm not about to do a full overhaul of the existing script if that's what it takes to add it.