Add a proposed API check option

[Tyriar]

Part of #2826

[jerch]

👍 LGTM.

I just wonder if collecting them under an .experimental property, that does this check once for all members, would be easier? And later on register the member on the main API as Terminal.XY, maybe with a transit mode for one or two version, where it is still reachable under Terminal.experimental.XY), so embedders have time to adopt.

While it would to separate things more clearly - not sure if we need this.

[Tyriar]

We could have both, there's an issue for that too. I really like a clear opt in this has where the apis just won't work until you flip the switch.

[Tyriar]

Also the still reachable under experimental thing puts extra burden on us, needing to keep 2 implementations for a bit. Less burden is what I'm going for 🙂

[jerch]

@Tyriar Yepp, the extra experimental stuff does not help much I guess, also if we'd go only with that, embedders have a hard time to fix the shifted symbols later on. So lets keep it simple.