import core in esm way

Expect to import core in esm way, like
```js
import { createFFmpeg, fetchFile } from '@ffmpeg/ffmpeg';
import ffmpegCore from '@ffmpeg/core;

createFFmpeg({ log: true, core: ffmpegCore });
```

Put string into require function is kind of confusing
https://github.com/ffmpegwasm/ffmpeg.wasm/blob/22b816091c577fb5e52c26b8ca7b968961980ba0/src/node/getCreateFFmpegCore.js#L6

Thanks,have a nice day!