Relocate Tab Loading to Frontend

[leeyi45]

Right now there are two steps to tab loading:

• js-slang first reads from tab files and then evaluates them
• Frontend calls that raw tab to turn it into a ModuleSideContent

The idea of a "Tab" is entirely a frontend concept: thus I propose that we move the loading of module side content into the frontend. I think this would do us some good by continuing the separate the responsibilities between js-slang and the frontend.