Godot 4 RC3 Type check and return fixes

[jaydensipe]

Godot introduced a new function is_instance_of to check if an Object is a type that is not known at runtime. This breaks functionality using is to check types not known at runtime. _put_packet_script function has also been updated to return Error instead of int to match parent function.

[CLAassistant]

All committers have signed the CLA.

[dsnopek]

Thanks!