Expose whether an `ArrayBuffer` is detachable

There is a pre-existing problem with ArrayBuffers in that it is not always possible to determine up-front whether or not a particular instance will succeed in being detached when asked to be transferred. Prior to this proposal, the notable case of an undetectable non-detachable buffer was the buffer exposed by WebAssembly.Memory. (This is as opposed to SharedArrayBuffer, which can be detected via instanceof.)

This proposal adds a new reason that an ArrayBuffer might not be detachable, and it requires a kind-specific check (.immutable) to make the detachability determination. And it seems likely that this won't be the last kind of buffer that won't be detachable.

I'd like to suggest that the fact of detachability be directly represented as a detachable: boolean property. It would be true exactly when a request to transfer() would succeed based on the detachability criterion. The choice of polarity here is intentional, in that an implementation without the property would fail-safe in terms of client code trying to decide between copy or transfer.

To be clear, it is currently possible to recover from a failed transfer with try-catch if one is trying to implement copy-or-transfer. However, that means that a correct case would have to accept the inefficiency of generating and catching an exception. And beyond that inefficiency, it does not make for particularly ergonomic code. And, in general, it's just plain useful to be able to make the determination without actually trying.

I actually ran into exactly this case (wrt WebAssembly.Memory) and during my search for a resolution ended up finding this proposal. So I figured it'd be worth raising the suggestion here. In my case, it was preferable to accept that my code will throw rather than silently degrade performance (punting the performance problem to the caller), but I'd really rather not be in the position to have to make that tradeoff.

首先审查 ArrayBuffer 当前的可分离性规则，以及本 issue 中描述的 transfer() 行为。评估一个可分离性布尔属性是否适合该提案，包括所述的极性和 WebAssembly.Memory 情况；完成的标准是得出并记录一项规范决策。