Rewrite conversion.py

It would be good to rewrite conversion.py based on syntax trees with proper parsers for python and javascript. On the python side, that would be the [ast module](https://docs.python.org/3/library/ast.html). I'm sure there are similar tools on the JavaScript side. I think it might also be good to have the conversion system be in a totally separate package that geemap depends on. Right now, the conversion module only depends on a few things from geemap and only from coreutils: random_string and temp_file_path https://github.com/gee-community/geemap/blob/683ac4214024129d3f5b76856a5bf228a956a632/geemap/conversion.py#L23 And example of using python's ast module: https://github.com/asottile/pyupgrade