Find a way to track UUIDs on VS Code editor objects

[phillco]

For splits, we're currently planning to use the editor path as the primary key for now, but eventually we would like to support having the same file open in multiple splits.

To do this most reliably, we will need to have UUIDs to represent editors that would be the same on both sides. Storing this information should be pretty trivial on the Emacs/JetBrains side, but we need a way to do so inside of Visual Studio Code.