ESM Build

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the feature has not already been requested

### 🚀 Feature Proposal

Ship an ESM build for json-schema-ref-resolver so this code could run in a browser (or edge worker) environment.

### Motivation

ESM is the new standard; packages should ship an ESM build for compatibility.

### Example

```ts
import { RefResolver } from 'json-schema-ref-resolver'
```