Add children roSGNode containers

[Christian-Holbrook]

Adds virtual variables for $count and $children for roSGNode and $count for arrays.

[TwitchBronBron]

Few changes:

• Let's align with a new convention of $ instead of [[ ]], (so $count)
• only add these virtual variables if the protocol version is <3.3

It might make sense to add a helper function or separate file/class to manage these, as I expect we'll be adding more of them in the future. That way the getVariablesResponse doesn't get too large.

[TwitchBronBron]

Looks really good! One comment.

[TwitchBronBron]

This looks great!