Give SharedArrayBuffer a dedicated type (downstream)

[annevk]

I wanted to create a tracking issue corresponding to PR #1311 as the issues it resolves aren't directly about that and it would be good to have a central place to coordinate all the downstream work.

Everything under [AllowShared] in https://dontcallmedom.github.io/webidlpedia/extended-attributes.html is impacted:

• [x] Encoding https://github.com/whatwg/encoding/pull/313
• [x] File System https://github.com/whatwg/fs/pull/128
• [ ] WebGL / WebGL 2.0 https://github.com/KhronosGroup/WebGL/pull/3539
• [ ] WebCodecs https://github.com/w3c/webcodecs/pull/683
• [ ] WebGPU https://github.com/gpuweb/gpuweb/pull/4188

I'm pretty sure that's it, but if people think there might be more let me know.

[bathos]

I think this change may incidentally fix some of the not-quite-right usages of “ArrayBuffer” (pointing at the Web IDL def) in WebCrypto.

[Edit: nm, still kinda busted there, but for unrelated reasons — will open an issue.]

[annevk]

I've done everything I can at this point. At least that I know about. I'll leave this open for a bit to see if there's any other fallout and maybe wait for all downstream PRs to have landed.