Use `Base.loaded_modules` instead of checking Main when loading JlrsCore

[asinghvi17]

This commit allows JlrsCore to be detected if it is loaded at any point in the given session, not just from main. I'm not sure if this is the best approach, but it's probably better than hard-checking if Main.JlrsCore is a thing.

[Taaitaaiger]

Thanks!

[Taaitaaiger]

Looks like a few quotes need to be escaped