Unstaking didn't actually work because it was using `transferFrom()` on the
`StakeVault` with the `from` address being the vault itself.
This would result in an approval error because the vault isn't creating
any approvals to spend its own funds.
The solution is to use `transfer` instead and ensuring the return value
is checked.
Because the `stakedToken` property is `immutable`, solhint recommends to
make it in all caps. This commit changes the property to adhere to that
rule and also makes the property private.
To access the `stakedToken` there's now a `stakedToken()` function on
the contract.
This commit migrates the repo to our foundry template, which ensures we
have consistent tooling across smart contract repositories that are
maintained by Vac.
This removes all hardhat related files and workflows and replaces them
with more perfomant foundry workflows.
It also sets up tests, CI and linting.
