Add polyfill for BigInt

[lukasIO]

Thanks first of all for this project, it's exactly what I was looking for!

This PR simply adds an option to polyfill BigInt to the rules.

[robatwilliams]

I didn't think BigInt could be polyfilled properly. This article agrees: https://javascript.info/bigint#polyfills

What are you using to polyfill it?

[robatwilliams]

Made me think - raised #71

[lukasIO]

A bit more context:
I'm running the compat check on the bundled output of a TypeScript project.
A third party dependency of the lib is doing some checks like

if(globalThis.BigInt !== undefined) {
  // ... proceeds to reference BigInt
}

that should be safe (as long as globalThis is actually polyfilled), but the es lint plugin complains.
So I'm trying to work around it.
Totally agree though that it's not really ideal as that would mask other occurences of BigInt to be wrongfully marked as polyfilled 🤔

[robatwilliams]

I see.

The problem of feature checks is a general one, but for all non-syntax features it can be worked around by stating that you have a polyfill (even if you don't actually). Except for things that aren't properly (or at all) polyfillable like BigInt here and probably ES2017 Atomics.

It might be possible to implement something here to guess if an expression looks like a feature test. We have the AST node there. This feels like the proper way to solve the wider problem, which I'd like to try.

In the meantime perhaps you could use your code changes here via patch-package ?

[lukasIO]

To have proper support for detecting feature tests sounds ideal!

Yeah, I'll work around it with a forked version for now, but very excited to see support for feature checks!

Thanks for your feedback, will close this PR.