[heycam/webidl] Change dictionary section to encourage Infra wording (#524)

Per https://github.com/whatwg/dom/issues/574 we no longer need to use 

>_dict_'s member is present 

and can instead use 

> _dict_["member"] exists

The dictionary section should probably encourage the latter style and only define "present" for as long as it takes everyone to migrate.

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