More flexible fenced code block generation

[Dansoftowner]

Overview

This PR makes fenced code block rendering more flexible in jpro-mdfx.

Previously, MDFXNodeHelper always created a MarkdownCodeBlock directly for fenced code blocks. This made it hard for users to customize code block rendering.

Changes

• Added MarkdownView.generateFencedCodeBlock(String code, String language).
• Updated fenced code block generation to call this new method instead of instantiating MarkdownCodeBlock directly.
• Kept the default behavior unchanged by returning new MarkdownCodeBlock(code, language) from the default implementation.

Why

This allows users to subclass MarkdownView and customize fenced code block rendering, similar to the existing customization points for links and images.

For example, users can now provide their own code block node, wrap the default MarkdownCodeBlock, add extra controls, change layout behavior, or apply app-specific behavior while keeping the default MDFX behavior intact.

Example

Let's say we want to add a context menu to each code block to give the user the ability to easily copy the particular code to the clipboard:

var markdownView = new MarkdownView("", markdownExtensions) {
            // ...

            @Override
            public Node generateFencedCodeBlock(String code, String language) {
                Node codeBlock = super.generateFencedCodeBlock(code, language);

                MenuItem copyItem = new MenuItem("Copy to Clipboard");
                copyItem.setOnAction(event -> {
                    ClipboardContent content = new ClipboardContent();
                    content.putString(code);
                    Clipboard.getSystemClipboard().setContent(content);
                });

                var contextMenu = new ContextMenu(copyItem);

                codeBlock.setOnMouseClicked(event -> {
                   if (event.getButton() == MouseButton.SECONDARY) {
                       contextMenu.show(codeBlock, event.getScreenX(), event.getScreenY());
                   }
                });

                return codeBlock;
            }
        };

It's now possible using the standard api.

Note: This example has been put to the sample application.