[heycam/webidl] Syntatictic way to mark as historical/deprecated (#910)

There are several "historical" things that can't be removed for compatibility but not recommended for modern web platform, for example `KeyboardEvent#charCode`.

So far spec authors write prose or comments to describe those as historical, but it's hard for TSJS-lib-generator to track such things. A syntactic way would be helpful here, for the above example something like:

```webidl
partial dictionary KeyboardEventInit {
  historical unsigned long charCode = 0;
  historical unsigned long keyCode = 0;
};
```

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