Format markdown with markdown formatter

[sheiun]

Is your feature request related to a problem? Please describe.
No relate to any problem.
In some editor we can use markdown formatter (like prettier) to format markdown documents.

Describe the solution you'd like
Integrate with prettier or another markdown formatter.

Describe alternatives you've considered
Copy content to local editor or online formatter to format documents and paste it back.

Additional context
Before

After

[Yukaii]

In CodiMD we used markdownlint for linting Markdown document.

The same package introduce vscode-extension with autofix functionaility. We might port this to HackMD & CodiMD

https://github.com/DavidAnson/vscode-markdownlint/blob/b596db721a7519bf87235d964c48c45f1078fab5/extension.js#L529-L551

https://github.com/DavidAnson/markdownlint/blob/main/helpers/README.md