Doesn't work with iOS emojis

This line https://github.com/peterhry/CircleType/blob/ffebbcc65803e1e203bde9522a8ff115de884b73/src/utils/splitNode.js#L13

```js
.split('')
```

wrongly splits emojis in iOS webview, if you change to

```js
for (const char of trimmedText) {
  // char
}
```

it will work with iOS emojis