Add Markstream to Streaming

[Simon-He95]

Checklist:

• I've read and understood Contributing Guidelines.
• I've added the new resource at the end of its section.
• This resource is out there for a while, and actively maintained.
• This resource is popular enough and has at least a few hundred stars on GitHub.

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Markstream is a documented, MIT-licensed JavaScript/TypeScript library for rendering incomplete Markdown while AI-chat responses stream. It supports Vue, React, Svelte, and Angular. I maintain the project.

[gokaygurcan]

you're an open source contributor and maintainer, please respect other people's pr templates would you?

[Simon-He95]

You’re right — I omitted the repository PR template. I’ve restored it and completed the checks against the contribution guide. Sorry for the oversight, and thank you for pointing it out.