It is written in D, a fast (close to C++) language, easy to write code in, runs in a terminal, runs graphically, runs on many systems (I don't know every possible system but I doubt in runs in a Commodore 64), it is open source and I have no idea about it being extensible or not.
It's however the closest thing I have seen to your list of requirements.
https://dlang.org/blog/2016/10/07/project-highlight-dlangui/
It is written in D, a fast (close to C++) language, easy to write code in, runs in a terminal, runs graphically, runs on many systems (I don't know every possible system but I doubt in runs in a Commodore 64), it is open source and I have no idea about it being extensible or not.
It's however the closest thing I have seen to your list of requirements.