Translate `src/posts/summary.js` to TypeScript and fix unit tests

[GustavoNazarioPerez]

These changes are a part of NodeBB's migration to TypeScript, resolves #29.

I have translated the file src/posts/summary.js from JavaScript to TypeScript, src/posts/summary.js now contains the compiled version of src/posts/summary.ts.

Key Changes:

• Added type annotations to src/posts/summary.js
• Created interfaces for the following objects: Post, CategoryObject, TopicObject, and UserObjectSlim. These objects were defined in the src/types directory but I could not import them as it would cause formatting errors due to the different module syntax (ESLint vs. CommonJS).
• Resolved a timeout issue in test/posts.js unit tests by removing the done callback.

Testing

• Checked for lint errors by running npm run lint
• Verified changes by running npm run test

[coveralls]

Pull Request Test Coverage Report for Build 7748333972

• -2 of 63 (96.83%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 85.096%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/posts/summary.js	61	63	96.83%
<!--	Total:	61	63	96.83%	-->

Totals
Change from base Build 7649102120:	0.003%
Covered Lines:	22634
Relevant Lines:	25362

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💛 - Coveralls