Add classes variable grouping

[jgutman]

there are a bunch of changes #1788 (comment) we discussed to the variable pane and its groupings, several of which probably require some discussion/alignment. One of the more straightforward ones is breaking off Classes as a distinct group from the catch-all Values. If I import a class from a module or submodule, or if I create my own class in the console, it will be grouped under Values, but this doesn't feel quite right. So along with the accompanying PR on the Python extension, adding a new category called Classes. (This could in theory be used in the R extension as well, but because of how R imports work, R classes won't show up in the Variables pane the way Python classes currently do, so probably less necessary. We will still need to commit the auto-generated changes to crates/amalthea/src/comm/variables_comm.rs though)