Adding Stella as A2600 emulation core

[SergioMartin86]

Adding Stella as A2600 emulator.

Progress has been painful and messy; don't expect minimalism during this first stage. We can do the fine tuning once everything works correctly.

Check if completed:

• I have run any relevant test suites

• I, the committer, have read the licensing terms for contributors (last updated 2024-03-20) and am compliant

• Load Rom

• Advancing Frame

• Processing Digital Inputs

• Processing Analog Inputs

• Rendering frame into the screen

• Rendering sound

• Memory Accesses

• CPU Hooks

[CasualPokePlayer]

Note, GPGX isn't particularly the best example of how to do a waterbox core, if it's at all possible use the standard WaterboxCore base class (see PicoDrive, TIC80, uzem, as simple examples of this being used)

[SergioMartin86]

Ok, opening this PR for review. This is a summary of what's been done:

• Emulation works correctly for several games I tested (including Myst, which apparently a2600hawk does not support)
• Video is rendering correctly
• Audio is also playing correctly, except for the first few frames or so (try Frogger (USA).a26)
• Statable interface correctly implemented
• Memory domains correctly showing
• Digital Controllers working

To-Dos:

• Analog controllers (paddle) not currently supported
• CPU hooks and debug functonality not current implemented

[nattthebear]

Can this be placed in the invisible section?

[SergioMartin86]

Yes, that'd make sense