[FEATURE] Include other-code source-ref in compiler error messages

[nachumg]

• [x ] I have reviewed this project's contribution guidelines

Describe the problem/limitation you think should be addressed
The following are error-messages, which only emit the source-ref to the duplicate (error-causing) code. All detected and reported in core/namespace.py

1. Multiple declarations of type
2. Multiple declarations of instance
3. Default property ... was already assigned in this scope

Describe the solution you'd like
Also report the source-ref of the already-existing declaration / property assignment.

Describe alternatives you've considered
None.

Additional context
None.

[amykyta3]

Thanks for the suggestion.

I am already planning on overhauling the message logging system (#168) and this would be a good feature to include along with that.

[amykyta3]

Fixed