Support custom Markdown renderer and HTML sanitizer

[jfx2006]

Allow for custom options in Marked and an HTML santizer.

Sample usage:

function mkRender() {
    const mdRender = new Marked()

    mdRender.setOptions({
      async: false,
      gfm: true,
      pedantic: false,
      breaks: false,
    })
    mdRender.use(
        markedHighlight({
          langPrefix: "hljs language-",
          highlight(code, lang) {
            const lowerLang = lang.toLowerCase()
            const language = hljs.getLanguage(lowerLang) ? lowerLang : "plaintext"
            return hljs.highlight(code, { language }).value
          },
        }),
    )

    return mdRender.parse
}

const editor = new MarkdownEditor('#markdown-editor', {
  placeholder: 'Write your markdown...',
  mode: "hybrid",
  toolbar: ['heading', 'bold', 'italic', 'strikethrough', 'ul', 'ol', 'checklist', 'blockquote', 'link', 'image', 'preview'],
  renderer: mkRender(),
  sanitizer: DOMPurify.sanitize,
});