Add conversion for SharedArrayBuffer

[MattiasBuelens]

As of whatwg/webidl#1311, WebIDL has its own SharedArrayBuffer type. I added a conversions.SharedArrayBuffer function to properly recognize this.

I also noticed that this conversion was incorrectly throwing an error, so I fixed that:

const converted = conversions.ArrayBuffer(new SharedArrayBuffer(0), { allowShared: true });

Questions:

• The old way of denoting this was [AllowShared] ArrayBuffer. However, that's no longer supported: you can only apply [AllowShared] on a buffer view type. For now, I've kept the allowShared option around in conversions.ArrayBuffer, but perhaps we should remove it instead?

[domenic]

• For now, I've kept the allowShared option around in conversions.ArrayBuffer, but perhaps we should remove it instead?

Removing it seems better to me!

[MattiasBuelens]

All right, I'll do that.

I'll keep the options parameter around though, since I still want to add support for [AllowResizable] in a follow-up PR. 😉

[domenic]

Let me know if you'd like me to cut a new release now, or wait for the [AllowResizable] support first.

[MattiasBuelens]

Opened #51. 😉