Fix workaround with GitHub latest styles

[hyrious]

This PR did various changes to make it work with the latest styles from GitHub.

• Fixed an oversight in previous versions where auto.css includes duplicated CSS variables. See the light section here: https://cdn.jsdelivr.net/npm/github-markdown-css@5.4.0/github-markdown.css
• Refactored works from Add support for alerts #26 to match the latest alert structures:

  <div class="markdown-alert markdown-alert-note" dir="auto">
    <p class="markdown-alert-title" dir="auto">
      <svg class="octicon octicon-info mr-2" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"></path></svg>
      Note
    </p>
    <p dir="auto">Highlights information that users should take into account, even when skimming.</p>
  </div>

  Which means it now adds classes like markdown-alert-note markdown-alert-title octicon-info mr-2 to the ALLOW_CLASS set.
• Refactored ALLOW_TAGS to match the latest sanitizer's code.
• Workaround new CSS syntax @container to make it able to parse GitHub CSS again.

To read the output diff:

$ node test
$ wget https://cdn.jsdelivr.net/npm/github-markdown-css/github-markdown.css
$ git diff --no-index github-markdown.css dist/auto.css

Next steps:

As I mentioned in #27, GitHub will use newer CSS syntaxes from now on -- they are using @container, maybe the next is @layer or even CSS nestings. We need not a workaround but a solution to parse these new CSS syntaxes. Maybe integrating lightningcss, or making a fork of the css module to add supports (it is not updated for 2 years).

[sindresorhus]

Maybe integrating lightningcss, or making a fork of the css module to add supports (it is not updated for 2 years).

I think we should switch. CSS doesn't stop evolving, so it doesn't make sense to continue maintenance of an abandoned package.