Implement IDL Float16Array

[annevk]

As per whatwg/webidl#1398.

[bartlomieju]

We probably need to update op2 macro to support these as well. CC @mmastrac

[mmastrac]

Do we have a v8 implementation CL to follow? We'll need to ship in rusty_v8 as well.

[bakkot]

V8 issue is here.

[petamoriken]

I'll work on this after #23490 is merged

Oops, op2 macro needs to be updated. I will work on it after that.

[marvinhagemeister]

Resolved by #23512