Multiple core support

[thenick775]

Back to the idea of what we had before in the original release, the ability to choose from multiple cores!

I'm evaluating the following gba cores:

• NanoBoyAdvance (preferred first integration)
• SkyEmu

Some things to consider:

• these cores use threads
• we will need to add headers for cross origin isolation
• can mGBA be threaded, and actually reap benefits in the browser?
• can we maintain a standard wasm contract as initially defined for the existing mGBA core?

This will take some time to implement, but is definitely doable.

My current priorities are tied up learning a new language at this time, so this may take a few months to bear any fruit. But I encourage anyone else interested in emacripten/wasm to take a crack at it!