feat($shared-utils): enhance header parsing

[Mister-Hope]

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

Linters (e.g. Prettier) will escape $ , e.g.:

## $accentColor

will be linted to

## \$accentColor

So \$ should be checked and unexcaped.

Also some people will use katex or mathjax, superscript and subscript to display math or chemistry expression in their headings. Though vuepress is not providing these plugins by default, it's better that vuepress can handle them

§ 1.1

Infomation about H₂O

## $\S$ 1.1

## Infomation about H~2~O