JS: Add support for vuex

[asgerf]

Adds support for vuex, a Flux-type state management library for Vue.

It is similar to Redux but was simpler to model as more of the core functionality seems to delivered out of the box, so things are a bit more consistent and fewer third-party libraries needed to be supported.

The model is based on API graphs, and there were a few tweaks needed to make this work nicely:

• API graphs can now backtrack def-nodes through spread operators.
• API graphs understand property accesses where the property name comes from an imported constant.
• String literals are now source nodes, so getAValueReachingRhs().getStringValue() can be used get string values reaching a def-node. (This landed separately in JS: Treat string literals as SourceNodes #6192)

The Vue model is not yet based on API graphs, so I only did a very loose integration, so as not to make the eventual migration harder.

Evaluations: (both containing a slightly older version of the string literals as source nodes PR causing some spurious alert diffs)

• Default slugs looks good
• On 30 vuex apps: discovers some more tainted nodes, but no new alerts

[erik-krogh]

Vuex.qll is quite a mouthful.
(Everything outside of Vuex.qll looks fine).

I'm not done reviewing it, but here preliminary review.

[asgerf]

Thanks for the initial review @erik-krogh, I know it's a heavy PR to work through. ✨

I've addressed the comments so far.

[erik-krogh]

A few minor comments, but nothing big.

Looks good 👍