Integrate with caniuse.com for easier legacy browsers support

[MaxDesiatov]

Motivation

As we start using newer features, like BigInt, FinalizationRegistry etc, we should still allow targeting older browsers.

Proposed solution

I'm currently thinking we could make --environment option more granular, allowing users to specify browser versions or JS versions, similarly to how --target option of esbuild works. Based on this information, we could pass additional flags to JSKit build (namely JAVASCRIPTKIT_WITHOUT_WEAKREFS), enable/disable i64 transform etc.

Alternatives considered

Not sure if direct support in SwiftPM and Swift for something like this is on the table yet, I previously tried something similar with https://github.com/swiftwasm/swift-package-manager/pull/373.