POL updates, migration to Foundry/Hardhat

[simonDos]

This merge includes 3 different major changes:

1. POL-related SC updates from plasma-bridge-update branch (reviewed and audited)
2. Migration of the whole repository and all tests to use Foundry/Hardhat (description below)
3. Deletion of deprecated/unused contracts (which are not on mainnet)

Migration details
We used the new Foundry template repo as a starting point, but incorporated Hardhat to easily re-use the old tests.
All test were then migrated to be used with new JS version and Hardhat framework

No Smart Contract was edited during this migration!

Goals:

• compiles with forge build
• compiles with hardhat compile
• all tests run with hardhat
• solidity coverage works
• README updated

type: module

As the old tests relied heavily on import statements, we decided to make the whole project "type": "module" .

This results in the necessary renaming of some config files like hardhat.config.cjs or solcover.cjs using the cjs extension.

Config

To maintain a single source of truth for our config, we import foundry.toml into hardhat.config.cjs to define solidity version and optimiser runs.