feat: add vega interpreter as dependency and support custom interpreter

[domoritz]

New version of #475

📦 Published PR as canary version: 6.18.3--canary.747.450ca4f.0

✨ Test out this PR locally via:

npm install vega-embed@6.18.3--canary.747.450ca4f.0
# or 
yarn add vega-embed@6.18.3--canary.747.450ca4f.0

[domoritz]

@stefanw could you review this pull request?

[stefanw]

Setting the interpreter sets ast to true by default.

Currently, this doesn't seem to be included.

I'm unsure what level of abstraction vega-embed is going for. An idea could be offer an option noeval (or similar) which could then set ast and expr appropriately behind the scenes. This makes it easy to configure CSP-compliance without having to learn about how vega actually achieves it. Or rephrased: as a vega-embed user, do I care that vega needs to generate an AST and pass it to an expression interpreter?

[domoritz]

Setting the interpreter sets ast to true by default.

Right, I removed that. I don't expect many people have to change the interpreter and if they set it, it doesn't mean they always want to use it (hence it shouldn't enable ast implicitly). I updated the comment.

Since expr is already set by default, setting ast is pretty much your proposed noeval. I agree that ast isn't at the right level of thinking for most users but since we already have the option, I can't remove it without making a breaking change.

We can reconsider the naming in the future but for now, I'll leave the names as they are. Hopefully, the documentation is good enough so that people find the option they need.

[github-actions]

🚀 PR was released in v6.19.0 🚀