Export ECMAScript modules (ESM)

[dretsa]

While CJS continues to be an important part of the NodeJS world, it seems to be slowly moving towards ESM. I think we should export both cjs and ems.

The reason for this is so that libraries using negotiator like tinyhttp can be fully ESM. I am happy to help with a PR :)

[dretsa]

I have created a new repository for an ESM/TS version of this package. Let me know what you think: https://github.com/colorninja/navigator-es

The branch is called ts-esm-port

Changed:

• Included lockfile (package-lock was ignored)
• Added TypeScript
• Changed package type to module
• Ported to TS
• Ensured exported types are the same or better than @types/negotiator
• Ensured exports are the same (default and named)
• Migrated tests to built-in node test module (reduce dependencies)
  • very easy because already using assert built-in module

Remaining to do:

• Add backwards compatibility for CJS
• Ensure package will be published with correct files
• Improve TS types