JS: Track functions through props and returns

[asgerf]

Addresses https://github.com/github/codeql-javascript-team/issues/18 by tracking functions through properties and returns, but not into calls (to avoid FPs from callbacks).

We avoid tracking anything that's a class member as those are tracked via the class instead. We also omit functions in externs for purely speculated performance reasons.

Evaluations:

• Security/smoke-test looks interesting, about 1% slowdown but with a surprising number of new call edges.
• Security/nightly also shows a lot of new call edges, but runtime was too flaky to be conclusive, even with the reruns it looks a little fishy.
• Security/big-apps times out on gecko.

TODO

• Investigate the TP rate of these call edges
• Larger evaluation

[asgerf]

It doesn't quite scale on gecko, and tweaking the join order doesn't seem to be resolve the issue. I'll close and try something less ambitious.