fix: ntl serve for v2 functions, follow-ups

[Skn0tt]

Contains two follow-ups to #6076, that were discovered while working on the Astro adapter.

1. ntl serve should pick up routes from manifest.json
2. __netlify-entrypoint.js is the wrong extension, it needs to be .mjs - see https://github.com/netlify/zip-it-and-ship-it/blame/f5fb309109408d25544fe34bdee77f917ff679e7/src/runtimes/node/utils/entry_file.ts#L150

[github-actions]

📊 Benchmark results

Comparing with 4027528

• Dependency count: 1,396 (no change)
• Package size: 404 MB (no change)
• Number of ts-expect-error directives: 0 (no change)