Add Mettle.js

[maomincoding]

Checklist:

• [x] I've read and understood Contributing Guidelines.
• [x] I've added the new resource at the end of its section.
• [x] 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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Mettle can be used as a MVC Framework/Library, and is used to build MVC Frameworks/Libraries.

GitHub Repository | Documentation

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[maomincoding]

Thank you very much. Mettle.js is more in line with development intuition, and you will feel comfortable during the development process.