`polkadot-omni-node`: re-namings

[kianenigma]

• [ ] Rename existing polkadot-parachain to polkadot-omni-node, and the existing polkadot-parachain-lib to polkadot-omni-node-lib.
• [ ] Then, Create a new polkadot-parachain that is now an instantiation of polkadot-omni-node-lib with system chain specs hardcoded.

A continuation of: https://github.com/paritytech/polkadot-sdk/issues/3944

It would be good to do this sooner rather than later to prevent breaking the names any longer.