refactor: rename .mts to .ts

[ericapisani]

Summary

Using the .mjs extension is no longer needed now that we've completed the transition to ESM. Having these file extensions is redundant since we have type:module in the package.json.

This change updates files that were previously .mts to .ts, which means that the compiled files will be .js. As a result, import lines needed to be updated as well.

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[github-actions]

📊 Benchmark results

Comparing with e5c33a0

• Dependency count: 1,396 (no change)
• Package size: 404 MB ⬇️ 0.00% decrease vs. e5c33a0
• Number of ts-expect-error directives: 0 ⬇️ Infinity% decrease vs. e5c33a0