add gtree

[ddddddO]

• [x] I've read CONTRIBUTING.md.
• [x] Description explains the issue / use-case resolved, and auto-closes the related issue(s) (https://help.github.com/articles/closing-issues-via-commit-messages/).

Web app that converts from markdown to tree using WebAssembly! 👇👇👇

site: https://ddddddo.github.io/gtree/ description: https://github.com/ddddddO/gtree#web

I would be happy to be merged👍

[Yee1023]

邮件我已收到，谢谢！！

[ddddddO]

@mbasso Hi👋 I hope you can see this PR!

[mbasso]

I am sorry for the very late reply Thank you for contributing!

[ddddddO]

Thanks for merging!