Editorial: Add an Assertion for Built-in Function Object

[kimjg1119]

In Construct method for built-in function object, it seems that the result must be an object. However, current ESMeta is unable to check this since BuiltinCallOrConstruct handles Call and Construct together.

It will be helpful to add an assertion to resolve a false alarm.

ESMeta alarm:

[ReturnTypeMismatch] Block[6404] return statement in Record[BuiltinFunctionObject].Construct (step 1, 2:12-96)
- expected: Normal[Record[Object]] | Throw
- actual  : Normal[ESValue] | Throw