Add support for 'LogPoints' in debugger

[DonJayamanne]

PTVS has this today, they call it Trace Points (hence functionality should be available in the new version of PTVSD).

Requires upstream changes as well #1306

[qubitron]

FYI this is currently planned for the March release for VS Code, microsoft/vscode#45128, but it might be at preview quality.

[DonJayamanne]

but it might be at preview quality.

Is this based on the issues/feedback on VS Code github repo?

[qubitron]

Per Kenneth's last comment in the upstream issue, he had some concerns with readiness.

[DonJayamanne]

@qubitron
I've managed to get this working in PTVSD (microsoft/ptvsd#350).
I think we can land this feature as well.