🐛 Fix TOC visible text and GitHub slug generation

[Lemmingh]

Summary

• Fix Exporting produces invalid HTML for fenced code blocks #662: The <div> tags are a quirk of vscode.markdown-language-features, which uses them for source mapping. But we don't need.
• Fix Incorrect TOC for reference link #515: This time, work on parsing result to simulate GitHub's rendering pipelines. We will no longer have any problem in generating slugs for it, unless markdown-it has bug.
• Fix TOC visible text generation. (Links to headings with emoji code may be broken #792 (comment) case 2)
• Fix slug generation for exporting in MarkdownEngine class.
• Introduce IDisposable, then we can create and register our own disposables in future.
• Normalize the interface of Markdown engines.
• Add getDocumentToken() to Markdown engines.
• Roll ECMAScript private fields back to TypeScript private properties. It occurred to me that five years ago, TS's transpiling for async functions caused a few surprising results. Since private field is still a stage 3 proposal, we'd better not adopt it now, for stability.
• Update webpack to 5.19.0 by the way.

Verification

Note:
Our heading recognition in TOC does not support multiline heading, so, you need to "Print to HTML" to inspect the last heading.
Be careful about those trailing spaces.

# `</title><script>alert('woo');</script><title>`

```c
#include <stdio>
// 我
```

```
<em>text</em>
```

##  ``b``[a]_c_[]\\[ [k](h) **g**![`m`](i) <https://r.u/> <small>d</small>  <!--p-->

e
f  
s\
&Aring;&#8629; &#27700; \&lt; <r>  😂 </r>
---

[a]: v

[yzhang-gh]

LGTM 👍.

Any thoughts about the failed test?

[Lemmingh]

That function is dedicated to link text, thus, only escaping inline syntax is enough. # cannot become an evil here.

As for the timeout ... no idea.