Add languages that compile to JS

[morags]

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.
• [x] This resource is popular enough and has at least a few hundred stars on GitHub.

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This PR adds a new section for JS alternatives and extensions. It lists both dedicated replacements/extensions like Elm, TypeScript and JSX, as well as transpilers for established languages like C#, Java and Python.

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