Add module and exports field to package.json

[lennondotw]

Summary

Add module and exports fields to package.json for proper ESM support.

Changes

• Add "module": "./esm/index.js" for bundler ESM resolution
• Add "exports" map covering root, ./esm, ./plugin/*, ./esm/plugin/*, ./locale/*, ./esm/locale/*
• Normalize main and types paths to use relative ./ prefix

Benefits

• Modern bundlers (Vite, webpack 5, Rollup) can resolve ESM entry points correctly
• Node.js conditional exports work for both import and require
• Fully backward compatible — no breaking changes

[haakonflatval-cognite]

@iamkun I'm seeing problems in some ESM-projects projects I'm working on that require extra patching of import paths from day.js in our config files due to malformed/missing export fields in the package.json, which is what this PR is trying to address. It would be greatly appreciated if you could consider merging this or one of the similar ones (e.g. https://github.com/iamkun/dayjs/pull/2543/files)

[lennondotw]

Rebased on latest dev, squashed to a single commit, and cleaned up the exports map — root entry now properly resolves ESM imports (no more "import": null), and removed the unintended packageManager field.

@iamkun would appreciate a look when you have a moment — this addresses the ESM import issues mentioned by @haakonflatval-cognite above.