feat: add implement markdown file preview in Ace editor

[ultwcz]

I like Filebrowser very much. I also deploy it on my server and use it almost every day. I often use Markdown files,

Reference the following link:
add mavon-editor for markdown file #1127

so I added a Markdown format preview function to it. Use marked to add Markdown support in Ace editor.

I am a novice in Go and JS. I hope you can give me some modification suggestions to verify whether my understanding is correct. Thank you very much.

[ultwcz]

@o1egl

Thank you for your review. I found some things I did not do well. I will pay special attention to them.

Thank you.

[baur]

@ultwcz

Hi, can you look this issue: #3461