fix: use correct file extensions for transpilation

[estephinson]

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Summary

When we did the mass .m(ts|js) -> .(js|ts) extension change these got caught up. This effected the function transpillation process as it was parsing the wrong file types, creating a nested functions-serve directory which borked the gatsby plugin

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📊 Benchmark results

Comparing with 7e3349c

• Dependency count: 1,399 (no change)
• Package size: 405 MB ⬇️ 0.00% decrease vs. 7e3349c
• Number of ts-expect-error directives: 1,331 (no change)