Export types in package.json

---

**Is your feature request related to a problem? Please describe.**

When installing `@googlemaps/three` and importing with typescript via `import { ThreeJSOverlayView } from '@googlemaps/three';`
I get an error `Could not find a declaration file for module '@googlemaps/three'. '.../google-maps-webgl/node_modules/@googlemaps/three/dist/index.esm.js' implicitly has an 'any' type.
There are types at '.../google-maps-webgl/node_modules/@googlemaps/three/dist/index.d.ts', but this result could not be resolved when respecting package.json "exports". The '@googlemaps/three' library may need to update its package.json or typings.`

**Describe the solution you'd like**
Export the `dist/index.d.ts` as well.

package.json before:
```json
"main": "dist/index.umd.js",
"exports": {
".": {
"import": "./dist/index.esm.js",
"require": "./dist/index.umd.js",
"umd": "./dist/index.umd.js",
"browser": "./dist/index.esm.js",
}
},
"unpkg": "dist/index.min.js",
```

package.json after:

```json
"main": "dist/index.umd.js",
"exports": {
".": {
"import": "./dist/index.esm.js",
"require": "./dist/index.umd.js",
"umd": "./dist/index.umd.js",
"browser": "./dist/index.esm.js",
"types": "./dist/index.d.ts"
}
},
"unpkg": "dist/index.min.js",
```

**Describe alternatives you've considered**
Making that change manually by editing the dependency inside my `node_modules` folder.