Implement stackTrace, scopes, variables and continue

[jtpio]

Add the following calls:

• stackTrace
• scopes
• variables
• continue

Also add the debug protocol interface to be able to use the correct types: https://github.com/microsoft/vscode-debugadapter-node/blob/master/protocol/src/debugProtocol.ts
This will be replaced by adding vscode-debugprotocol as a dependency when jupyterlab/jupyterlab#6704 is merged and released.

[jtpio]

Adding a (very) simple temporary variables viewer:

It's only for Locals for now. Once we decided how we want to render them, we can pass the full variables json / dict to the renderer directly.