Allow to use custom unified visitors.

[aexol]

https://github.com/aexol-studio/mdtx/blob/main/packages/mdtx-core/src/transformers/markdown.ts
In this file and in the nextJS plugin we should have a possibility to insert our own unified plugins.

 const processedContent = await unified()
    .use(remarkParse)
    .use(remarkDirective)
    .use(() => {
      return (tree, file) => {
        visit(tree, 'containerDirective', (node) => {
          const data = node.data || (node.data = {});
          const hast = h(node.name, node.attributes);
          data.hName = hast.tagName;
          data.hProperties = hast.properties;
        });
        visit(tree, 'html', (node) => {
          if (node.value.includes('src="')) {
            node.value = node.value.replace(
              /src\=\".*\.gitbook/,
              'src="/.gitbook',
            );
            // tree.children.push({ type: '' });
          }
        });
        visit(tree, 'paragraph', (node) => {
          const image = node.children.find((child) => child.type === 'image');
          if (image && image.type === 'image') {
            const fileName = image.url.replace(/^.*\.gitbook/, '/.gitbook');
            image.url = fileName;
          }
        });
      };
    })
    .use(remarkRehype, { allowDangerousHtml: true })
    .use(rehypeRaw)
    .use(rehypeHighlight)
    .use(rehypeStringify)
    .process(replaceCustomTags);

[aexol]

We should add a possibility to provide array of unified plugins