[Feature] Copy to clipboard functionality for Code Block component

[Jettonn]

Initial checklist

• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Problem

Code Block component is really useful, but since it's created not to be extended by the user it's hard to add a copy-to-clipboard functionality to the component (although you can change the icons, buttons, etc for language selector).

Solution

It would be handy to be able to add functionalities yourself (extend the components) or just support copy to the clipboard button.

Alternatives

Let users extend milkdown components and plugins

[sensiblearts]

I'm trying to copy the current selection as markdown.

     theEditor.editor.action((ctx: Ctx) => {
            const editorView = ctx.get(editorViewCtx);
            let selection = editorView.state.tr.selection;
            // now what...?
            // I searched through `selection`'s fields, recursively,
            // but could not find a method to get the markdown.      
    });

Perhaps, something like selection.getMarkdown() would solve mine and would subsume your code block copy?

Then it's just a matter of navigator.clipboard.write(), isn't it?