types(dia.Paper): fix elementView and linkView paper option

[kumilingus]

Description

1. Make the elementView (and linkView) types more general to allow passing custom views without the need for casting.

Before:

new dia.Paper({
 elementView: CustomElementView as typeof dia.ElementView
})

After:

new dia.Paper({
  elementView: CustomElementView
})

2. Allow elementView callback to return null or undefined (to signalize using the default view).

Before:

new dia.Paper({
 elementView: (element) => 
      if (element.get('type) === 'CustomElement') {
        return CustomElementView;
      }
      // otherwise use the default view (either one from namespace or `dia.ElementView`).
      return null;
   }
})

[Copilot]

Pull request overview

This PR improves the type definitions for the elementView and linkView options in dia.Paper to support custom view classes without requiring explicit type casting. The changes allow these options to accept any subclass of ElementView or LinkView through generic type parameters, and enable callback functions to return null or undefined to indicate use of default views.

Changes:

• Updated type definitions for elementView and linkView options to use generic type parameters (<any>)
• Modified callback return types to allow null or undefined for fallback to default views
• Added test cases demonstrating the new functionality with custom view classes

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/joint-core/types/joint.d.ts	Updated Paper.Options type definitions for elementView and linkView to support generic views and nullable callback returns
packages/joint-core/test/ts/index.test.ts	Added CustomLinkView class and test cases for both direct class assignment and conditional callback with null return

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