Add return type hint for Encoder::encode() implementation

This was not applicable to the commit in v1.x (36fca42), but is necessary in v2.x since 5dda098 added return types to all methods.

Author: jmikola

tests/Builder/BuilderEncoderTest.php

@@ -393,7 +393,7 @@ public function canEncode(mixed $value): bool
                     return $value instanceof FieldPathInterface;
                 }
 
-                public function encode(mixed $value)
+                public function encode(mixed $value): mixed
                 {
                     return '$prefix.' . $value->name;
                 }