get ArrayBuffer.prototype.immutable: Missing check for SharedArrayBuffers

[anba]

https://tc39.es/proposal-immutable-arraybuffer/#sec-get-arraybuffer.prototype.immutable is missing the step to reject SharedArrayBuffers:

3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.

SharedArrayBuffers should be rejected to match the other ArrayBuffer.prototype getters. https://github.com/tc39/test262/pull/4445/files#diff-e0871ee6e58cc12a83ebd6a7c577c26bc5db55f9bb2a0baeacbe21084191a3de also expects that passing a SharedArrayBuffer throws.

[gibson042]

Thanks for the report; opened #51 to fix.