Add Sound

- Should play a tone, with frequency configurable in header files, if the register `st` is > 0
- Preferably uses SDL, since we're already using that, if there's an easier solution go ahead, but try and keep it cross paltform compatibly
- If you're using an audio clip, make sure it doesnt break while looping, I've heard that that's sometimes a problem
- Add a test rom src in `rom_src/` to make sure it works

Look at the existing code to see how SDL is used and where the `st` register is defined. The header files need a configurable frequency. Create a test ROM in `rom_src/` that exercises the sound. Ensure the sound loops correctly without breaking.

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