fix: Add exports for module compatibility

[2wheeh]

There are some open PRs for ESM support but apart from that it needs proper exports to be resolved by modern bundlers.

Once it supports dual format (esm, cjs), it just need to update like:

  "exports": {
    ".": {
      "types": "./index.d.ts",
      "import": "./index.mjs",
      "require": "./index.js"
    },
    "./*": {
      "types": "./*.d.ts",
      "import": "./*.mjs",
      "require": "./*.js"
    }
  },

[dynst]

These fields will be completely ignored once "exports" exists so they don't actually do anything.

[dynst]

You can just put one line "default" here for now instead.

But this all conflicts with the PR for adding ESM support #104