This PR is part of the process separation project.
Move NodeImpl from interfaces/node.cpp to node/interfaces.cpp
Move ChainImpl from interfaces/chain.cpp to node/interfaces.cpp
Move WalletImpl from interfaces/wallet.cpp to wallet/interfaces.cpp
No changes to any classes (can review with git diff --color-moved=dimmed_zebra)
Motivation for this change is to move node and wallet code to respective directories where it might fit in better than src/interfaces/, but also to remove all unnecessary code from src/interfaces/ to unblock #19160 review, which has been hung up partially because of code organization. Building on top of this PR, #19160 should now be able to organize interface implementations more understandably in src/node/ src/wallet/ src/ipc/ and src/init/ directories instead of having so much functionality all in src/interfaces/