Replace markdown parser from Greenmat to Qiita Marker

[kyntk]

Currently, Qiita Markdown uses Greenmat, an extension of Redcarpet, to parse Markdown.
On February 21st, 2022, we changed the Markdown parser used in Qiita to Qiita Marker, an extension of CommonMarker.

Accordingly, this gem also changes its Markdown parser from Greenmat to Qiita Marker.

Please see the following page for details about changes on Qiita.
https://blog.qiita.com/replace-markdown-parser-general/

Effects of the Markdown parser change on rendering results

The basic parsing results will remain the same, however some notations will be changed and rendering results may change.

After the change, it will be GitHub Fravored Markdown Spec compliant.
https://github.github.com/gfm/

The following is a list of some of the relatively commonly used ones in Qiita.

Headings: Need at least one space after #

Bad

##Headings previously used

Good

## Having at least one space after marker

HTML blocks: Need at least one blank line after HTML block

<details><summary>Summary</summary>

## Need at least one blank line before following markdown

Paragraph.
</details>

Also need it above this line, too.

Emphasis and strong emphasis

Content to be emphasized may vary depending on the presence or absence of spaces before and after the word to be emphasized.
Put a space before and after, e.g., "plain text **text to be emphasized** plain text".

The others

Please see the following page for details.
https://help.qiita.com/ja/articles/qiita-markdown#markdown