[Feature request] GFM support in @vuepress/markdown

[Mister-Hope]

Clear and concise description of the problem

I receive many users complaining that they are unable to use our tools to build a existing markdown folder in their project.

Mainly because we do not have full support of GFM.

It's good for us to support "standard" markdown files in GitHub.

Suggested solution

Add a new gfm option

Alternative

use vuepress-plugin-md-enhance with { gfm: true } option

[meteorlxy]

I'm planning to extract those md-it plugins to a standalone project

[Mister-Hope]

Any update with this one?

[Mister-Hope]

This one also needs review. @meteorlxy Answer these questions plz:

1. Shall the GFM feature be added to @vuepress/markdown or a new plugin called @vuepress/plugin-gfm? OR just close this pr to let vuepress-plugin-md-enhance support this?

2. What kind of GFM features shall we support? Additional thing including:

FootNote:

Footnote 1 link¹.

Footnote 2 link².

Inline footnote^[Text of inline footnote] definition.

Duplicated footnote reference².

Footnote 1 link[^first].

Footnote 2 link[^second].

Inline footnote^[Text of inline footnote] definition.

Duplicated footnote reference[^second].

[^first]: Footnote **can have markup**

    and multiple paragraphs.

[^second]: Footnote text.

Tasklists:

• item
• item2
• item3

- [ ] item
- [x] item2
- [X] item3

Warning and note hints:

Note
Some text here

Note: Some text here

Note: Some text here
Addtional Content

Warning
Some text here

Warning: Some text here

Warning: Some text here
Addtional Content

> **Note**
> Some text here

> **Note**: Some text here

> **Note**: Some text here
> Addtional Content

> **Warning**
> Some text here

> **Warning**: Some text here

> **Warning**: Some text here
> Addtional Content

Rendering math fence and $$ blocks with mathjax

$$ a = 1 $$

```math
a = 1
```

$$
a = 1
$$

```math
a = 1
```

Mermaid blocks with mermaid:

---
title: Flowchart
---
flowchart TB
    c1-->a2
    subgraph one
    a1-->a2
    end
    subgraph two
    b1-->b2
    end
    subgraph three
    c1-->c2
    end
    one --> two
    three --> two
    two --> c2

```mermaid
---
title: Flowchart
---
flowchart TB
    c1-->a2
    subgraph one
    a1-->a2
    end
    subgraph two
    b1-->b2
    end
    subgraph three
    c1-->c2
    end
    one --> two
    three --> two
    two --> c2
```

Image mark:

![GitHub Light](/assets/image/github-light.svg#gh-dark-mode-only)
![GitHub Dark](/assets/image/github-dark.svg#gh-light-mode-only)

Some of the features would generate large chunks, so what do we need to support?

Footnotes

1. Footnote can have markup

   and multiple paragraphs. ↩

2. Footnote text. ↩ ↩²