All editor features should consume attributes they put in the model during upcast

[oleq]

📝 Provide detailed reproduction steps (if any)

In #11530 we came across an issue with GHS (double-)consuming the src of an image and creating a mess. The issue was crazy but the fix pretty straightforward.

While fixing this my gut told me there could be other features that also touch attributes in the upcast pipeline but not consume them, which would spawn dozens of #11530 clones in the future. 

To check this, I created a simple plugin

function( editor ) {
    editor.data.upcastDispatcher.on( 'element', ( evt, data, conversionApi ) => {
        console.group( `element:${ data.viewItem.name }` );

        for ( const [ key ] of data.viewItem.getAttributes() ) {
            if ( conversionApi.consumable.test( data.viewItem, { attributes: key } ) ) {
                console.warn( `Looks like "${ key }" was not consumed.`, data.viewItem );
            }
        }

        console.groupEnd();
    }, { priority: 'lowest' } );
} 

and ran it against the all-features manual test. You can add it straight to plugins: [] or better yet: put it in the implementation of Paragraph because it's everywhere and you can then run just any test (manual or automatic). 

I discovered there are plenty of plugins that leave things unconsumed, these are just a few examples:

CodeBlock

Using code blocks + GHS + text direction

<span lang="plaintext" dir="undefined"><pre><code class="language-plaintext">foo</code></pre></span>

MediaEmbed

TextPartLanguage

ImageInline

Image again

More Image

We need to review all of them at patch them up because otherwise we'll hit it again and again while working on GHS. Just keep in mind that some attributes may not need to be consumed if they were ignored by the feature (not transferred to the model).

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[oleq]

Side thought: Could my plugin be used to test things automatically? Or as a dev tool? In the inspector?

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