Add a variable explorer

[josevalim]

In a given code cell, we should be able list all variables available alongside their type and a snippet of their values. Perhaps with even a search bar to filter it. In the future, once we provide exploration rich visualization to some of them, we can incorporate it too. :) Here is a snapshot of how a similar feature works on Deepnote:

[josevalim]

Closing this as it is now tracked in a meta issue.