Migrate from git submodules to Soldeer

[fadeev]

Soldeer is a Solidity package manager integrated with forge: https://soldeer.xyz/

I propose we:

• Install everything that's available from npm
• Install everything else from Soldeer
• If we use a Soldeer dependency in one of the contracts in the contracts/ dir, we add this dependency in git, so that when protocol contracts are installed through npm it still works.
• Not use git submodules

It's not great to have two package managers, but some things like forge-std are not available on npm.

We can't not use npm, because ABIs need to be published there for frontend clients to use.