[w3c/editing] contenteditabledisabled is extending `ElementContentEditable` in a wrong way (#332)

[ElementContentEditable](https://html.spec.whatwg.org/multipage/interaction.html#elementcontenteditable) is an existing mixin, but the spec somehow is [redefining it as an interface](https://w3c.github.io/editing/docs/contentEditableDisabled/#element-contenteditable-mixin).

This is wrong and it should be `partial interface mixin` instead, as noted in https://github.com/w3c/editing/pull/265#discussion_r476025740.

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