Add agent support for the Node v10 runtime

[juliobbv]

Package the Node v10 runtime with the agent, and add a new Node10 handler that can be referenced by the task's task.json.

Additionally, add a feature flag to switch existing Node handler to use either the v6 or the v10 runtimes.

[TingluoHuang]

@juliobbv LGTM, let me know when you finish all testing and ready to merge. :)

[juliobbv]

@juliobbv LGTM, let me know when you finish all testing and ready to merge. :)

Thanks @TingluoHuang. I finished all of my testing and everything looks good.

[damccorm]

This is a continuation of my comment here where I pointed out a place node 10 doesn't honor back-compat, but this seems like a better location for this conversation. I stumbled across another instance of fs being used in a way that is incompatible with node 10, but this time it was in existing task code. I did a quick search of tasks containing fs and after looking at about 25% of them found 4 different instances of fs used in ways that are incompatible with node 10. It will throw a runtime error (so building doesn't catch it).

The places I found them are:

• Common/code-coverage-tools
• Common/packaging-common
• 2 instances of Kubernetes V0 (instance 1 and instance 2)

I'd guess there's more instances of this since I only looked at about 25% of tasks containing fs. I'm not sure what the best path forward is here, but it feels like we should be updating those tasks before we turn this on by default