Add htmlNormalization event to ClipboardPipeline.

[Mati365]

Suggested merge commit message (convention)

Feature (clipboard): It's now possible to attach custom HTML serialize functions to clipboard pipeline, just before processing it into view fragment. Closes #17309
Internal (paste-from-office): Use inputHtmlNormalization clipboard event to handle HTML normalization.

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Additional information

The ClipboardPipeline is performing some HTML normalization (on string) just before converting it to the view document fragment:

ckeditor5/packages/ckeditor5-clipboard/src/clipboardpipeline.ts

Lines 229 to 233 in d874050

	if ( dataTransfer.getData( 'text/html' ) ) {
	contentData = normalizeClipboardHtml( dataTransfer.getData( 'text/html' ) );
	} else if ( dataTransfer.getData( 'text/plain' ) ) {
	contentData = plainTextToHtml( dataTransfer.getData( 'text/plain' ) );
	}

• There is no easy extension point to add additional normalizers.
• The PasteFromOffice is ignoring the prepared view document fragment and is performing a separate normalization on the data transfer payload:
  

  ckeditor5/packages/ckeditor5-paste-from-office/src/pastefromoffice.ts

  Line 83 in d874050

  const htmlString = data.dataTransfer.getData( 'text/html' );

  
  

  ckeditor5/packages/ckeditor5-paste-from-office/src/pastefromoffice.ts

  Line 88 in d874050

  data._parsedData = parseHtml( htmlString, viewDocument.stylesProcessor );

  
  Those cases could be unified as an extension point in the ClipboardPipeline so non-parsed data could be processed before conversion to the view document fragment.

~Kuba

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⚠️ I'm not pretty sure if this flag is still needed: 79555ac#diff-4b980c1afbffe5148af9316676ffda4ac77a98664dc69fcf7c07ba7726d3c42cR47. It's not fully documented so I kept it in evt.return property.

[niegowski]

The ClipboardPipeline is a linear sequence of events (as described at the top of this file). This change would make it inconsistent as it would be a side quest. I suggest extracting the clipboardInput firing code from the ClipboardObserver to the corresponding pipelines (clipboard pipeline and dragdrop). Then the normalization could be implemented in the listener for the paste (and drop) event listener and the sequence of events would not be broken. Especially that copy and cut are implemented locally in the pipeline, not in the observer.