More supports of MathJax3

[Liam0205]

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features). see Update math-equations.md theme-next.org#86

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build related changes.
• CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

Issue resolved: N/A

Related issues/pull requests: #1314

#1314 introduced support of MathJax3, which should be shown appreciation to. This PR

• loads the amsmath package, who is the standard-in-reality of math-typesetting in LaTeX-world, by default;
• translates `$ ... $`, `\( ... \)`, `$$ ... $$`, and `\begin{foo} ... \end{bar}` into specific span, and waiting for MathJax's processing, when theme.math.mathjax.codemath is set to be true.

What is the new behavior?

• Screenshots with this changes: N/A
• Link to demo site with this changes: N/A

How to use?

N/A

Does this PR introduce a breaking change?

• Yes.
• No.

[stevenjoezhang]

You can write the implementation of this feature as a blog post and provide a link here:
https://github.com/next-theme/awesome-next#tutorial

I'm not sure how many users need this feature. Maybe you can start a poll to investigate? I am worried that writing mathematical formulas in code blocks is just a workaround, not a general solution. This can be misleading for new users.