Provide helper to split a short signature's `vs`

We have a function tryRecover(hash, r, vs) to support short signatures, but sometimes you want to receive a short signature and split it into a long signature without doing the recover operation.

For example a function that uses permit could look like this:

function doThingWithPermit(..., uint256 value, uint256 deadline, bytes32 r, bytes32 vs) {
      (uint8 v, bytes32 s) = ECDSA.splitShort(vs);

      try token.permit(msg.sender, address(this), value, deadline, v, r, s) {} catch {}

      doThing(..., value);
}

token.permit expects v, r, s, but doThingWithPermit can be much more calldata efficient if it receives vs.

Start at the existing ECDSA.tryRecover(hash, r, vs) implementation and its ECDSA entry point. Add the requested splitShort(vs) helper so callers can obtain v and s for token.permit, then add or update tests to verify the returned components and confirm the permit use case.