Add back "gas-custom-errors" in solhint

[gjermundgaraba]

In #107, we had to turn off "gas-custom-errors" because solhint did not understand the new require syntax (which allows custom errors in require in a gas efficient way).

We want to add this rule back as soon as solhint supports it. The next (current is 5.0.3) release of solhint will likely include this fix: protofire/solhint#602