[heycam/webidl] Define an operation to make a promise as handled (#829)

Split from https://github.com/whatwg/fetch/pull/959.

Streams and Fetch sometimes manipulate promise.\[[PromiseIsHandled]] to suppress unhandled rejection error reports. We'd appreciate if this is defined here so that they don't need to manipulate the internal property directly.

@annevk, @domenic, @ricea

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