LibJS: Make String.prototype.slice() generic

Libraries/LibJS/Runtime/StringPrototype.cpp

@@ -388,12 +388,10 @@ Value StringPrototype::includes(Interpreter& interpreter)
 
 Value StringPrototype::slice(Interpreter& interpreter)
 {
-    auto* string_object = string_object_from(interpreter);
-    if (!string_object)
+    auto string = string_from(interpreter);
+    if (string.is_null())
         return {};
 
-    auto& string = string_object->primitive_string().string();
-
     if (interpreter.argument_count() == 0)
         return js_string(interpreter, string);
 

Libraries/LibJS/Tests/String.prototype-generic-functions.js

@@ -16,6 +16,7 @@ try {
         "concat",
         "substring",
         "includes",
+        "slice",
     ];
 
     genericStringPrototypeFunctions.forEach(name => {