Detect if in VSCODE notebook environment

[yifanwu]

This is a duplicate of microsoft/vscode-python#16034

Re-opening here b/c I didn't find the solution to work.

I tried what the holoviz folks did in my vscode notebook and didn't find any of the env vars (in fact no env vars contained the substring "VSCODE" https://github.com/holoviz/panel/blob/9ea8c34d2029f6ed6a486aa2360566e7d16ca405/panel/config.py#L849

Appreciate the help!