Support Node 12

[nolanmar511]

Right now, this is blocked because binaries won't build when using 12.3.0 (because of nodejs/node#27803), and the Node 12 alpine image currently uses 12.3.0.

But, in a couple days, I assume that will be sorted out, and otherwise pprof-nodejs seems to be working with Node 12.

[nolanmar511]

The latest node:12-alpine image uses 12.3.1.

So this is unblocked there.

[nolanmar511]

The agent has been released since #70, so prebuilt binaries are available.

However, we're still blocked on setting up unit testing by googleapis/google-cloud-node#2923.

I re-purposed this issue to be a bit more general and capture supporting Node 12 in pprof-nodejs (which, I think, requires getting unit tests working).