fix: types in root dir

[xiaoxiangmoe]

since ts 4.5, typescript will read exports field in package.json

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

So we should keep type in root dir same as types in dist dir.