VS Code extension: Add IntelliSense suggestions in debug console

Nothing currently assists users as they type out expressions in the VS Code debug console. Not sure if there's already a feature the C# extension could hook into. But whether or not VS Code needs a new feature as well, this would be a good improvement to the debugging story.

BTW the ability to expand objects in the debug console output window helps but isn't sufficient. For one thing, method calls aren't supported there.