feat: kernel hover

[kiyoon]

Features

• :JupyniumKernelHover (default binding <space>K): Inspect a variable from the jupyter kernel
  • Shout out to https://github.com/lkhphuc/jupyter-kernel.nvim for providing the code base
    
• :JupyniumStartAndAttachToServerInTerminal: Useful for debugging

Fixes

• JupyniumKernelSelect crash when escape

Breaking changes

• Some default keybindings have changed. The default bindings are provided mainly for new users, and they may change to more sensible ones in the future. You should disable default bindings if you don't want this change!