[heycam/webidl] Disallow [LegacyArrayClass] on globals (#545)

This allows removing step 3 of the [create a named properties object algorithm](https://pr-preview.s3.amazonaws.com/tobie/webidl/pull/494.html#create-a-named-properties-object).

There are no such cases of globals which also have the [LegacyArrayClass] extended attribute, so that shouldn't be an issue.

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