adapter-netlify: Use CJS entrypoint to load ESM files

[GrygrFlzr]

• Rename ESM files to .mjs
• Add CJS index.js entry point for lambda

[Rich-Harris]

I feel a bit conflicted about this — it feels rather hacky. I almost wonder if adapters should be responsible for bundling the outputted ESM (this would also solve the question of what needs to be considered external during build, so that adapters that target lambda can create a single CJS file.

Alternatively maybe Vite could output both ESM and CJS from build.

I don't think it should hold us up though, I'll merge this and #483 and raise an issue for that