[heycam/webidl] Fix internal methods that only target platform objects w/ indexed/named props (#202)

The algorithm's introductory prose incorrectly limits the algorithm streps to platform objects that support indexed/named props, for example:

> The internal [[Delete]] method of every platform object O that implements an interface which supports indexed or named properties must behave as follows when called […].

Instead, the method should apply to *all* platform objects and see its algorithm steps forked to address the cases of platform objects which support indexed or named properties.

In addition, the common case should rely on the relevant ES `Ordinary[X]` method rather than a custom default behavior solution.

The following methods are affected:

* [[Set]]
* [[DefineOwnProperty]]
* [[Delete]]
* [[Call]]


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