[LeGo] Implement local-variables

Documentation:
https://auronen.cokoliv.eu/gmc/zengin/scripts/extenders/lego/tools/locals/

Original implementation from LeGo is based on string-builder, what is far from ideal. I believe it will be a lot cleaner to implement the feature on VM level.
Currently have working implementation (stack based) locally - will start PR, once string-pool feature is merged.