Implement default values for well-known property completions

[tintoy]

These will go in help/properties.json alongside the description attribute for each property. For example:

"AllowUnsafeBlocks": {
        "description": "Allow unsafe code blocks.",
        "defaultValue": "true"
}

CC: @doggy8088

[tintoy]

Someday, we may even offer a selection of default values for a property (which will become a snippet whose field has a select list for its value) but for now let's keep it simple :)