dillon1000/react

[react-native] Pull up enableFastAddPropertiesInDiffing check (#33043)

## Summary

We don't need the isArray check for this experiment, as
`fastAddProperties` already does the same. Also renaming
slowAddProperties to make it clearer we can fully remove this codepath
once fastAddProperties is fully rolled out.

## How did you test this change?

```
yarn test packages/react-native-renderer -r=xplat --variant=true
```