beacon/engine: move core/beacon to beacon/engine (#26616)

This PR moves core/beacon to beacon/engine so that beacon-chain related code has its own top level package which also can house the the beacon lightclient-code.

Author: zsfelfoldi

core/beacon/errors.go renamed to beacon/engine/errors.go

@@ -14,7 +14,7 @@
 // You should have received a copy of the GNU Lesser General Public License
 // along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
 
-package beacon
+package engine
 
 import (
 	"github.com/ethereum/go-ethereum/common"

core/beacon/gen_blockparams.go renamed to beacon/engine/gen_blockparams.go

@@ -14,7 +14,7 @@
 // You should have received a copy of the GNU Lesser General Public License
 // along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
 
-package beacon
+package engine
 
 import (
 	"fmt"