Fix workaround with GitHub latest styles

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 #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 / generate-github-markdown-css

Fix workaround with GitHub latest styles #28